diff --git a/src/analytics.ts b/src/analytics.ts
index 90cab9b..9cde53b 100644
--- a/src/analytics.ts
+++ b/src/analytics.ts
@@ -10,9 +10,9 @@ export function raiseError(error: Error) {
 
 export const withErrorLog: Middleware = function withErrorLog(method, args) {
   return [
-    async function() {
+    async function(...args: any[]) {
       try {
-        await method.apply(this, arguments)
+        await method.apply(null, args)
       } catch (error) {
         raiseError(error)
       }
@@ -21,9 +21,13 @@ export const withErrorLog: Middleware = function withErrorLog(method, args) {
   ]
 }
 
-function encodeParams(params: object) {
+function encodeParams<
+  T extends {
+    [key: string]: any
+  }
+>(params: T) {
   return Object.keys(params)
-    .map((key: keyof object) => `${key}=${encodeURIComponent(JSON.stringify(params[key]))}`)
+    .map((key: keyof T) => `${key}=${encodeURIComponent(JSON.stringify(params[key]))}`)
     .join('&')
 }
 
diff --git a/src/components/FileExplorer.tsx b/src/components/FileExplorer.tsx
index 9aead51..674f999 100644
--- a/src/components/FileExplorer.tsx
+++ b/src/components/FileExplorer.tsx
@@ -14,16 +14,13 @@ export type Props = {
   metaData: MetaData
   freeze: boolean
   compressSingletonFolder: boolean
-  accessToken: string
+  accessToken: string | undefined
   toggleShowSettings: React.MouseEventHandler
 }
 
 class FileExplorer extends React.Component<Props & ConnectorState> {
   static defaultProps: Partial<Props & ConnectorState> = {
-    treeData: null,
-    metaData: null,
     freeze: false,
-    visibleNodes: null,
   }
 
   componentWillMount() {
@@ -60,7 +57,7 @@ class FileExplorer extends React.Component<Props & ConnectorState> {
           <Node
             key={node.path}
             node={node}
-            depth={depths.get(node)}
+            depth={depths.get(node) || 0}
             focused={focusedNode === node}
             expanded={expandedNodes.has(node)}
             onClick={onNodeClick}
@@ -86,7 +83,7 @@ class FileExplorer extends React.Component<Props & ConnectorState> {
         className={cx(`file-explorer`, { freeze })}
         tabIndex={-1}
         onKeyDown={handleKeyDown}
-        onClick={freeze ? toggleShowSettings : null}
+        onClick={freeze ? toggleShowSettings : undefined}
       >
         {stateText ? (
           <LoadingIndicator text={stateText} />
diff --git a/src/components/SideBar.tsx b/src/components/SideBar.tsx
index a595042..2598d0a 100644
--- a/src/components/SideBar.tsx
+++ b/src/components/SideBar.tsx
@@ -80,7 +80,8 @@ class Gitako extends React.PureComponent<Props & ConnectorState> {
         {metaData && <MetaBar metaData={metaData} />}
         {errorDueToAuth
           ? this.renderAccessDeniedError()
-          : metaData && (
+          : metaData &&
+            treeData && (
               <FileExplorer
                 compressSingletonFolder={compressSingletonFolder}
                 toggleShowSettings={toggleShowSettings}
diff --git a/src/driver/connect.ts b/src/driver/connect.ts
index bf92a46..193f215 100644
--- a/src/driver/connect.ts
+++ b/src/driver/connect.ts
@@ -50,7 +50,9 @@ export type Dispatch<Props, State> = {
   call: TriggerOtherMethod<Props, State>
 }
 
-export type MethodCreator<Props, State, Args> = (dispatch: Dispatch<Props, State>) => Method<Args>
+export type MethodCreator<Props, State, Args = []> = (
+  dispatch: Dispatch<Props, State>,
+) => Method<Args>
 
 type Sources<P, S> = {
   [key: string]: MethodCreator<P, S, any>
diff --git a/src/driver/core/FileExplorer.ts b/src/driver/core/FileExplorer.ts
index 13de683..175d7e0 100644
--- a/src/driver/core/FileExplorer.ts
+++ b/src/driver/core/FileExplorer.ts
@@ -28,7 +28,12 @@ function getVisibleParentNode(nodes: TreeNode[], focusedNode: TreeNode, depths:
   const focusedNodeIndex = nodes.indexOf(focusedNode)
   const focusedNodeDepth = depths.get(focusedNode)
   let indexOfParentNode = focusedNodeIndex - 1
-  while (indexOfParentNode !== -1 && depths.get(nodes[indexOfParentNode]) >= focusedNodeDepth) {
+  let depth: number | undefined
+  while (indexOfParentNode !== -1) {
+    depth = depths.get(nodes[indexOfParentNode])
+    if (depth === undefined || focusedNodeDepth === undefined || !(depth >= focusedNodeDepth)) {
+      break
+    }
     --indexOfParentNode
   }
   const parentNode = nodes[indexOfParentNode]
@@ -44,13 +49,13 @@ const init: MethodCreator<Props, ConnectorState> = dispatch => () =>
 
 function resolveGitModules(root: TreeNode, blobData: BlobData) {
   if (blobData) {
-    if (blobData.encoding === 'base64') {
+    if (blobData.encoding === 'base64' && blobData.content && Array.isArray(root.contents)) {
       const content = atob(blobData.content)
       const parsed = ini.parse(content)
       Object.values(parsed).map((value: { url: string; path: string }) => {
         const { url, path } = value
         // for now, handle modules at root only
-        const node = root.contents.find(node => node.path === path)
+        const node = Array.isArray(root.contents) && root.contents.find(node => node.path === path)
         if (node) {
           node.url = url
         }
@@ -66,6 +71,7 @@ const setUpTree: MethodCreator<Props, ConnectorState> = dispatch => () =>
     const { root, gitModules } = treeParser.parse(treeData, metaData)
 
     if (gitModules) {
+      if (metaData.userName && metaData.repoName && gitModules.sha) {
       const blobData = await GitHubHelper.getBlobData({
         userName: metaData.userName,
         repoName: metaData.repoName,
@@ -160,14 +166,16 @@ const handleKeyDown: MethodCreator<
           if (focusedNode.type === 'tree') {
             if (expandedNodes.has(focusedNode)) {
               const nextNode = nodes[focusedNodeIndex + 1]
-              if (depths.get(nextNode) > depths.get(focusedNode)) {
+              const d1 = depths.get(nextNode)
+              const d2 = depths.get(focusedNode)
+              if (d1 !== undefined && d2 !== undefined && d1 > d2) {
                 dispatch.call(focusNode, nextNode, false)
               }
             } else {
               dispatch.call(setExpand, focusedNode, true)
             }
           } else if (focusedNode.type === 'blob') {
-            DOMHelper.loadWithPJAX(focusedNode.url)
+            if (focusedNode.url) DOMHelper.loadWithPJAX(focusedNode.url)
           } else if (focusedNode.type === 'commit') {
             window.open(focusedNode.url)
           }
@@ -177,7 +185,7 @@ const handleKeyDown: MethodCreator<
           if (focusedNode.type === 'tree') {
             dispatch.call(setExpand, focusedNode, true)
           } else if (focusedNode.type === 'blob') {
-            DOMHelper.loadWithPJAX(focusedNode.url)
+            if (focusedNode.url) DOMHelper.loadWithPJAX(focusedNode.url)
           } else if (focusedNode.type === 'commit') {
             window.open(focusedNode.url)
           }
@@ -231,13 +239,14 @@ const delayExpandThreshold = 400
 function shouldDelayExpand(node: TreeNode) {
   return (
     visibleNodesGenerator.visibleNodes.expandedNodes.has(node) &&
+    Array.isArray(node.contents) &&
     node.contents.length > delayExpandThreshold
   )
 }
 
 const setExpand: MethodCreator<Props, ConnectorState, [TreeNode, boolean]> = dispatch => (
   node,
-  expand = false
+  expand = false,
 ) => {
   visibleNodesGenerator.setExpand(node, expand)
   const applyChanges = () => dispatch.call(focusNode, node, false)
@@ -251,7 +260,7 @@ const setExpand: MethodCreator<Props, ConnectorState, [TreeNode, boolean]> = dis
 
 const toggleNodeExpansion: MethodCreator<Props, ConnectorState, [TreeNode, boolean]> = dispatch => (
   node,
-  skipScrollToNode
+  skipScrollToNode,
 ) => {
   visibleNodesGenerator.toggleExpand(node)
   const applyChanges = () => {
@@ -285,7 +294,7 @@ const onNodeClick: MethodCreator<Props, ConnectorState, [TreeNode]> = dispatch =
     dispatch.call(toggleNodeExpansion, node, true)
   } else if (node.type === 'blob') {
     dispatch.call(focusNode, node, true)
-    DOMHelper.loadWithPJAX(node.url)
+    if (node.url) DOMHelper.loadWithPJAX(node.url)
   } else if (node.type === 'commit') {
     window.open(node.url, '_blank')
   }
diff --git a/src/driver/core/SideBar.ts b/src/driver/core/SideBar.ts
index b9bc1dc..ec127a8 100644
--- a/src/driver/core/SideBar.ts
+++ b/src/driver/core/SideBar.ts
@@ -180,7 +180,7 @@ const setShouldShow: MethodCreator<
   ConnectorState,
   [ConnectorState['shouldShow']]
 > = dispatch => shouldShow => {
-  dispatch.set({ shouldShow }, shouldShow ? DOMHelper.focusFileExplorer : null)
+  dispatch.set({ shouldShow }, shouldShow ? DOMHelper.focusFileExplorer : undefined)
   DOMHelper.setBodyIndent(shouldShow)
 }
 
diff --git a/src/utils/DOMHelper.ts b/src/utils/DOMHelper.ts
index f1936ec..c839a0e 100644
--- a/src/utils/DOMHelper.ts
+++ b/src/utils/DOMHelper.ts
@@ -28,7 +28,7 @@ function setBodyIndent(shouldShowGitako: boolean) {
   }
 }
 
-function $<E extends (element: Element) => any, O extends () => any>(
+function $<EE extends Element, E extends (element: EE) => any, O extends () => any>(
   selector: string,
   existCallback?: E,
   otherwise?: O,
@@ -41,7 +41,7 @@ function $<E extends (element: Element) => any, O extends () => any>(
   : ReturnType<O> | ReturnType<E> {
   const element = document.querySelector(selector)
   if (element) {
-    return existCallback ? existCallback(element) : element
+    return existCallback ? existCallback(element as EE) : element
   }
   return otherwise ? otherwise() : null
 }
@@ -60,16 +60,15 @@ function getBranches() {
 function getCurrentBranch() {
   const selectedBranchButtonSelector =
     '.repository-content > .file-navigation > .branch-select-menu > button'
-  const branchNameFromButtonElement = $(
-    selectedBranchButtonSelector,
-    (element: HTMLButtonElement) => element.title.trim(),
+  const branchNameFromButtonElement = $(selectedBranchButtonSelector, element =>
+    (element as HTMLButtonElement).title.trim(),
   )
   if (branchNameFromButtonElement) return branchNameFromButtonElement
 
   const selectedBranchSelector =
     '.select-menu.branch-select-menu .select-menu-modal .select-menu-list .select-menu-item.selected svg.select-menu-item-icon + span'
   const branchNameFromSelectElement = $(selectedBranchSelector, element =>
-    element.textContent.trim(),
+    element.textContent ? element.textContent.trim() : '',
   )
   if (branchNameFromSelectElement) return branchNameFromSelectElement
 }
@@ -306,7 +305,7 @@ function attachCopySnippet() {
            *    </div>
            *  </article>
            */
-          target.parentNode.insertBefore(clippy, target)
+          if (target.parentNode) target.parentNode.insertBefore(clippy, target)
         }
       }
     }),
@@ -318,13 +317,18 @@ function attachCopySnippet() {
  */
 function focusFileExplorer() {
   const sideBarContentSelector = '.gitako-side-bar .file-explorer'
-  $(sideBarContentSelector, (sideBarElement: HTMLElement) => sideBarElement.focus())
+  $(sideBarContentSelector, sideBarElement => {
+    if (sideBarElement instanceof HTMLElement) sideBarElement.focus()
+  })
 }
 
 function focusSearchInput() {
   const searchInputSelector = '.search-input'
-  $(searchInputSelector, (searchInputElement: HTMLElement) => {
-    if (document.activeElement !== searchInputElement) {
+  $(searchInputSelector, searchInputElement => {
+    if (
+      document.activeElement !== searchInputElement &&
+      searchInputElement instanceof HTMLElement
+    ) {
       searchInputElement.focus()
     }
   })
diff --git a/src/utils/GitHubHelper.ts b/src/utils/GitHubHelper.ts
index 641ba17..ce1010e 100644
--- a/src/utils/GitHubHelper.ts
+++ b/src/utils/GitHubHelper.ts
@@ -21,7 +21,7 @@ type Options = {
 }
 
 async function request(url: string, { accessToken }: Options = {}) {
-  const headers = {} as {
+  const headers = {} as HeadersInit & {
     Authorization?: string
   }
   if (accessToken) {
@@ -76,7 +76,7 @@ async function getTreeData({ userName, repoName, branchName, accessToken }: Meta
 export type ItemData = {
   userName: string
   repoName: string
-  accessToken: string
+  accessToken?: string
 }
 
 export type BlobData = {
@@ -98,7 +98,7 @@ async function getBlobData({
 function getUrlForRedirect(
   { userName, repoName, branchName }: MetaData,
   type = 'blob',
-  path?: string
+  path?: string,
 ) {
   return `https://github.com/${userName}/${repoName}/${type}/${branchName}/${path}`
 }
diff --git a/src/utils/configHelper.ts b/src/utils/configHelper.ts
index 4df8ee9..d506dc7 100644
--- a/src/utils/configHelper.ts
+++ b/src/utils/configHelper.ts
@@ -2,8 +2,8 @@ import storageHelper from 'utils/storageHelper'
 import { pick } from 'utils/general'
 
 type Config = {
-  shortcut: string | null
-  access_token: string | null
+  shortcut: string | undefined
+  access_token: string | undefined
   compressSingletonFolder: boolean
   copyFileButton: boolean
   copySnippetButton: boolean
@@ -18,8 +18,8 @@ export enum configKeys {
 }
 
 const defaultConfigs = {
-  shortcut: null,
-  access_token: null,
+  shortcut: undefined,
+  access_token: undefined,
   compressSingletonFolder: true,
   copyFileButton: true,
   copySnippetButton: true,