diff --git a/src/vs/editor/common/modes.ts b/src/vs/editor/common/modes.ts
index 890b6b14584065eb98028a2f6182f48d5b692e26..53131ce3c41654c3f399d26301e816c8e7a615f0 100644
--- a/src/vs/editor/common/modes.ts
+++ b/src/vs/editor/common/modes.ts
@@ -1203,7 +1203,7 @@ export interface Command {
  * @internal
  */
 export interface CommentInfo {
-	extensionId: string;
+	extensionId?: string;
 	threads: CommentThread[];
 	commentingRanges?: (IRange[] | CommentingRanges);
 	reply?: Command;
@@ -1266,8 +1266,8 @@ export interface CommentThread2 {
 	comments: Comment[];
 	onDidChangeComments: Event<Comment[]>;
 	collapsibleState?: CommentThreadCollapsibleState;
-	input: CommentInput;
-	onDidChangeInput: Event<CommentInput>;
+	input: CommentInput | undefined;
+	onDidChangeInput: Event<CommentInput | undefined>;
 	acceptInputCommand?: Command;
 	additionalCommands: Command[];
 	onDidChangeAcceptInputCommand: Event<Command>;
@@ -1368,8 +1368,8 @@ export interface CommentThreadChangedEvent {
  */
 export interface DocumentCommentProvider {
 	provideDocumentComments(resource: URI, token: CancellationToken): Promise<CommentInfo>;
-	createNewCommentThread(resource: URI, range: Range, text: string, token: CancellationToken): Promise<CommentThread>;
-	replyToCommentThread(resource: URI, range: Range, thread: CommentThread, text: string, token: CancellationToken): Promise<CommentThread>;
+	createNewCommentThread(resource: URI, range: Range, text: string, token: CancellationToken): Promise<CommentThread | null>;
+	replyToCommentThread(resource: URI, range: Range, thread: CommentThread, text: string, token: CancellationToken): Promise<CommentThread | null>;
 	editComment(resource: URI, comment: Comment, text: string, token: CancellationToken): Promise<void>;
 	deleteComment(resource: URI, comment: Comment, token: CancellationToken): Promise<void>;
 	startDraft?(resource: URI, token: CancellationToken): Promise<void>;
diff --git a/src/vs/workbench/api/electron-browser/mainThreadComments.ts b/src/vs/workbench/api/electron-browser/mainThreadComments.ts
index 62ac83c48a2f5f3af40a90d7ce0359b3a79e0204..844cb64183d20b17f38eb144dad5019a9c3e2381 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadComments.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadComments.ts
@@ -127,7 +127,7 @@ export class MainThreadCommentThread implements modes.CommentThread2 {
 	}
 
 	get acceptInputCommand(): modes.Command {
-		return this._acceptInputCommand;
+		return this._acceptInputCommand!;
 	}
 
 	private _onDidChangeAcceptInputCommand = new Emitter<modes.Command>();
@@ -249,7 +249,7 @@ export class MainThreadCommentController {
 	}
 
 	deleteCommentThread(commentThreadHandle: number) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		this._threads.delete(commentThreadHandle);
 
 		this._commentService.updateComments(this._uniqueId, {
@@ -263,7 +263,7 @@ export class MainThreadCommentController {
 	}
 
 	updateComments(commentThreadHandle: number, comments: modes.Comment[]) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		thread.comments = comments;
 
 		this._commentService.updateComments(this._uniqueId, {
@@ -275,27 +275,27 @@ export class MainThreadCommentController {
 	}
 
 	updateAcceptInputCommand(commentThreadHandle: number, acceptInputCommand: modes.Command) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		thread.acceptInputCommand = acceptInputCommand;
 	}
 
 	updateAdditionalCommands(commentThreadHandle: number, additionalCommands: modes.Command[]) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		thread.additionalCommands = additionalCommands;
 	}
 
 	updateCollapsibleState(commentThreadHandle: number, collapseState: modes.CommentThreadCollapsibleState) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		thread.collapsibleState = collapseState;
 	}
 
 	updateCommentThreadRange(commentThreadHandle: number, range: IRange) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		thread.range = range;
 	}
 
 	updateCommentThreadLabel(commentThreadHandle: number, label: string) {
-		let thread = this._threads.get(commentThreadHandle);
+		let thread = this.getKnownThread(commentThreadHandle);
 		thread.label = label;
 	}
 
@@ -309,11 +309,21 @@ export class MainThreadCommentController {
 		}
 	}
 
+	private getKnownThread(commentThreadHandle: number) {
+		const thread = this._threads.get(commentThreadHandle);
+		if (!thread) {
+			throw new Error('unknown thread');
+		}
+		return thread;
+	}
+
+
 	async getDocumentComments(resource: URI, token) {
-		let ret = [];
+		let ret: modes.CommentThread2[] = [];
 		for (let thread of keys(this._threads)) {
-			if (this._threads.get(thread).resource === resource.toString()) {
-				ret.push(this._threads.get(thread));
+			const commentThread = this._threads.get(thread)!;
+			if (commentThread.resource === resource.toString()) {
+				ret.push(commentThread);
 			}
 		}
 
@@ -338,9 +348,9 @@ export class MainThreadCommentController {
 	}
 
 	getAllComments(): MainThreadCommentThread[] {
-		let ret = [];
+		let ret: MainThreadCommentThread[] = [];
 		for (let thread of keys(this._threads)) {
-			ret.push(this._threads.get(thread));
+			ret.push(this._threads.get(thread)!);
 		}
 
 		return ret;
@@ -540,17 +550,17 @@ export class MainThreadComments extends Disposable implements MainThreadComments
 					keys(this._workspaceProviders).forEach(handle => {
 						this._proxy.$provideWorkspaceComments(handle).then(commentThreads => {
 							if (commentThreads) {
-								const providerId = this._handlers.get(handle);
+								const providerId = this.getHandler(handle);
 								this._commentService.setWorkspaceComments(providerId, commentThreads);
 							}
 						});
 					});
 
 					keys(this._commentControllers).forEach(handle => {
-						let threads = this._commentControllers.get(handle).getAllComments();
+						let threads = this._commentControllers.get(handle)!.getAllComments();
 
 						if (threads.length) {
-							const providerId = this._handlers.get(handle);
+							const providerId = this.getHandler(handle);
 							this._commentService.setWorkspaceComments(providerId, threads);
 						}
 					});
@@ -564,6 +574,13 @@ export class MainThreadComments extends Disposable implements MainThreadComments
 		}
 	}
 
+	private getHandler(handle: number) {
+		if (!this._handlers.has(handle)) {
+			throw new Error('Unknown handler');
+		}
+		return this._handlers.get(handle)!;
+	}
+
 	$registerWorkspaceCommentProvider(handle: number, extensionId: ExtensionIdentifier): void {
 		this._workspaceProviders.set(handle, undefined);
 
@@ -611,7 +628,7 @@ export class MainThreadComments extends Disposable implements MainThreadComments
 
 	$unregisterDocumentCommentProvider(handle: number): void {
 		this._documentProviders.delete(handle);
-		const handlerId = this._handlers.get(handle);
+		const handlerId = this.getHandler(handle);
 		this._commentService.unregisterDataProvider(handlerId);
 		this._handlers.delete(handle);
 	}
@@ -627,14 +644,14 @@ export class MainThreadComments extends Disposable implements MainThreadComments
 			}
 		}
 
-		const handlerId = this._handlers.get(handle);
+		const handlerId = this.getHandler(handle);
 		this._commentService.removeWorkspaceComments(handlerId);
 		this._handlers.delete(handle);
 	}
 
 	$onDidCommentThreadsChange(handle: number, event: modes.CommentThreadChangedEvent) {
 		// notify comment service
-		const providerId = this._handlers.get(handle);
+		const providerId = this.getHandler(handle);
 		this._commentService.updateComments(providerId, event);
 	}
 
@@ -658,7 +675,10 @@ export class MainThreadComments extends Disposable implements MainThreadComments
 	async provideWorkspaceComments(): Promise<modes.CommentThread[]> {
 		const result: modes.CommentThread[] = [];
 		for (const handle of keys(this._workspaceProviders)) {
-			result.push(...await this._proxy.$provideWorkspaceComments(handle));
+			const result = await this._proxy.$provideWorkspaceComments(handle);
+			if (Array.isArray(result)) {
+				result.push(...result);
+			}
 		}
 		return result;
 	}
diff --git a/src/vs/workbench/api/node/extHost.protocol.ts b/src/vs/workbench/api/node/extHost.protocol.ts
index 15a16ca11cc2ed70ccd0d2cfa3ad43484bc383f9..4af7ec3763a3dbe23097e2ccf2795cde5c439b6e 100644
--- a/src/vs/workbench/api/node/extHost.protocol.ts
+++ b/src/vs/workbench/api/node/extHost.protocol.ts
@@ -1106,7 +1106,7 @@ export interface ExtHostProgressShape {
 export interface ExtHostCommentsShape {
 	$provideDocumentComments(handle: number, document: UriComponents): Promise<modes.CommentInfo>;
 	$createNewCommentThread(handle: number, document: UriComponents, range: IRange, text: string): Promise<modes.CommentThread | null>;
-	$onCommentWidgetInputChange(commentControllerHandle: number, input: string): Promise<number | undefined>;
+	$onCommentWidgetInputChange(commentControllerHandle: number, input: string | undefined): Promise<number | undefined>;
 	$provideCommentingRanges(commentControllerHandle: number, uriComponents: UriComponents, token: CancellationToken): Promise<IRange[] | undefined>;
 	$createNewCommentWidgetCallback(commentControllerHandle: number, uriComponents: UriComponents, range: IRange, token: CancellationToken): void;
 	$replyToCommentThread(handle: number, document: UriComponents, range: IRange, commentThread: modes.CommentThread, text: string): Promise<modes.CommentThread | null>;