Resolve more ESLint (#922)

Co-authored-by: Steve Faulkner <southpolesteve@gmail.com>

src/Explorer/Notebook/NotebookComponent/VirtualCommandBarComponent.tsx

@@ -1,5 +1,5 @@
-import * as React from "react";
 import { AppState, ContentRef, selectors } from "@nteract/core";
+import * as React from "react";
 import { connect } from "react-redux";
 import * as NteractUtil from "../NTeractUtil";
 

src/Explorer/Notebook/NotebookComponent/contents/file/index.tsx

@@ -2,7 +2,6 @@ import { AppState, ContentRef, selectors } from "@nteract/core";
 import * as React from "react";
 import { connect } from "react-redux";
 import styled from "styled-components";
-
 import NotebookRenderer from "../../../NotebookRenderer/NotebookRenderer";
 import * as TextFile from "./text-file";
 
@@ -32,14 +31,14 @@ interface FileProps {
 
 export class File extends React.PureComponent<FileProps> {
   getChoice = () => {
-    let choice = null;
+    let choice;
 
     // notebooks don't report a mimetype so we'll use the content.type
     if (this.props.type === "notebook") {
       choice = <NotebookRenderer contentRef={this.props.contentRef} />;
     } else if (this.props.type === "dummy") {
-      choice = null;
-    } else if (this.props.mimetype == null || !TextFile.handles(this.props.mimetype)) {
+      choice = undefined;
+    } else if (this.props.mimetype === undefined || !TextFile.handles(this.props.mimetype)) {
       // This should not happen as we intercept mimetype upstream, but just in case
       choice = (
         <PaddedContainer>

src/Explorer/Notebook/NotebookComponent/contents/file/text-file.tsx

@@ -1,10 +1,10 @@
-import * as StringUtils from "../../../../../Utils/StringUtils";
 import { actions, AppState, ContentRef, selectors } from "@nteract/core";
 import { IMonacoProps as MonacoEditorProps } from "@nteract/monaco-editor";
 import * as React from "react";
 import { connect } from "react-redux";
 import { Dispatch } from "redux";
 import styled from "styled-components";
+import * as StringUtils from "../../../../../Utils/StringUtils";
 
 const EditorContainer = styled.div`
   position: absolute;
@@ -37,7 +37,7 @@ interface TextFileState {
 class EditorPlaceholder extends React.PureComponent<MonacoEditorProps> {
   render(): JSX.Element {
     // TODO: Show a little blocky placeholder
-    return null;
+    return undefined;
   }
 }
 
@@ -98,7 +98,7 @@ function makeMapStateToTextFileProps(
 
     return {
       contentRef,
-      mimetype: content.mimetype != null ? content.mimetype : "text/plain",
+      mimetype: content.mimetype !== undefined ? content.mimetype : "text/plain",
       text,
     };
   };