diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 23c1332397f0bdc0376cc0719d8b2adc4b9cddb1..e8eea983c9f516a304eb1808b5e7bf284177a192 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -33,6 +33,7 @@
 		"./vs/workbench/services/timer/**/*.ts",
 		"./vs/workbench/contrib/webview/**/*.ts",
 		"./vs/workbench/contrib/debug/common/**/*.ts",
+		"./vs/workbench/contrib/debug/node/**/*.ts",
 		"./vs/workbench/contrib/preferences/common/**/*.ts",
 		"./vs/workbench/contrib/preferences/**/settings*.ts",
 		"./vs/workbench/contrib/terminal/**/*"
@@ -239,18 +240,9 @@
 		"./vs/workbench/contrib/debug/browser/debugStatus.ts",
 		"./vs/workbench/contrib/debug/browser/exceptionWidget.ts",
 		"./vs/workbench/contrib/debug/browser/linkDetector.ts",
+		"./vs/workbench/contrib/debug/browser/baseDebugView.ts",
 		"./vs/workbench/contrib/debug/browser/statusbarColorProvider.ts",
-		"./vs/workbench/contrib/debug/common/debug.ts",
-		"./vs/workbench/contrib/debug/common/debugProtocol.d.ts",
-		"./vs/workbench/contrib/debug/common/debugSchemas.ts",
-		"./vs/workbench/contrib/debug/common/debugSource.ts",
-		"./vs/workbench/contrib/debug/common/debugUtils.ts",
-		"./vs/workbench/contrib/debug/common/debugViewModel.ts",
 		"./vs/workbench/contrib/debug/electron-browser/rawDebugSession.ts",
-		"./vs/workbench/contrib/debug/node/debugAdapter.ts",
-		"./vs/workbench/contrib/debug/node/debugger.ts",
-		"./vs/workbench/contrib/debug/node/telemetryApp.ts",
-		"./vs/workbench/contrib/debug/node/terminals.ts",
 		"./vs/workbench/contrib/debug/test/common/debugSource.test.ts",
 		"./vs/workbench/contrib/debug/test/common/debugUtils.test.ts",
 		"./vs/workbench/contrib/debug/test/common/mockDebug.ts",
diff --git a/src/vs/workbench/contrib/debug/common/debug.ts b/src/vs/workbench/contrib/debug/common/debug.ts
index 000458a22fc3ca43a985e2a48c9f19f4bb5cb8c5..c6e4b73f1f0ad3856d28fe5f8c22811516780ea3 100644
--- a/src/vs/workbench/contrib/debug/common/debug.ts
+++ b/src/vs/workbench/contrib/debug/common/debug.ts
@@ -356,30 +356,30 @@ export interface IExceptionInfo {
 
 export interface IViewModel extends ITreeElement {
 	/**
-	 * Returns the focused debug session or null if no session is stopped.
+	 * Returns the focused debug session or undefined if no session is stopped.
 	 */
-	readonly focusedSession: IDebugSession;
+	readonly focusedSession: IDebugSession | undefined;
 
 	/**
-	 * Returns the focused thread or null if no thread is stopped.
+	 * Returns the focused thread or undefined if no thread is stopped.
 	 */
-	readonly focusedThread: IThread;
+	readonly focusedThread: IThread | undefined;
 
 	/**
-	 * Returns the focused stack frame or null if there are no stack frames.
+	 * Returns the focused stack frame or undefined if there are no stack frames.
 	 */
-	readonly focusedStackFrame: IStackFrame;
+	readonly focusedStackFrame: IStackFrame | undefined;
 
-	getSelectedExpression(): IExpression;
-	getSelectedFunctionBreakpoint(): IFunctionBreakpoint;
-	setSelectedExpression(expression: IExpression): void;
-	setSelectedFunctionBreakpoint(functionBreakpoint: IFunctionBreakpoint): void;
+	getSelectedExpression(): IExpression | undefined;
+	getSelectedFunctionBreakpoint(): IFunctionBreakpoint | undefined;
+	setSelectedExpression(expression: IExpression | undefined): void;
+	setSelectedFunctionBreakpoint(functionBreakpoint: IFunctionBreakpoint | undefined): void;
 
 	isMultiSessionView(): boolean;
 
 	onDidFocusSession: Event<IDebugSession | undefined>;
-	onDidFocusStackFrame: Event<{ stackFrame: IStackFrame, explicit: boolean }>;
-	onDidSelectExpression: Event<IExpression>;
+	onDidFocusStackFrame: Event<{ stackFrame: IStackFrame | undefined, explicit: boolean }>;
+	onDidSelectExpression: Event<IExpression | undefined>;
 }
 
 export interface IEvaluate {
diff --git a/src/vs/workbench/contrib/debug/common/debugViewModel.ts b/src/vs/workbench/contrib/debug/common/debugViewModel.ts
index 8296124dfb916db91b5820852c7201dbeb3de8df..9f0eb34aca631d568b8304ef4e17f6b40ee6ca66 100644
--- a/src/vs/workbench/contrib/debug/common/debugViewModel.ts
+++ b/src/vs/workbench/contrib/debug/common/debugViewModel.ts
@@ -11,14 +11,14 @@ export class ViewModel implements IViewModel {
 
 	firstSessionStart = true;
 
-	private _focusedStackFrame: IStackFrame;
-	private _focusedSession: IDebugSession;
-	private _focusedThread: IThread;
-	private selectedExpression: IExpression;
-	private selectedFunctionBreakpoint: IFunctionBreakpoint;
+	private _focusedStackFrame: IStackFrame | undefined;
+	private _focusedSession: IDebugSession | undefined;
+	private _focusedThread: IThread | undefined;
+	private selectedExpression: IExpression | undefined;
+	private selectedFunctionBreakpoint: IFunctionBreakpoint | undefined;
 	private readonly _onDidFocusSession: Emitter<IDebugSession | undefined>;
-	private readonly _onDidFocusStackFrame: Emitter<{ stackFrame: IStackFrame, explicit: boolean }>;
-	private readonly _onDidSelectExpression: Emitter<IExpression>;
+	private readonly _onDidFocusStackFrame: Emitter<{ stackFrame: IStackFrame | undefined, explicit: boolean }>;
+	private readonly _onDidSelectExpression: Emitter<IExpression | undefined>;
 	private multiSessionView: boolean;
 	private expressionSelectedContextKey: IContextKey<boolean>;
 	private breakpointSelectedContextKey: IContextKey<boolean>;
@@ -38,19 +38,19 @@ export class ViewModel implements IViewModel {
 		return 'root';
 	}
 
-	get focusedSession(): IDebugSession {
+	get focusedSession(): IDebugSession | undefined {
 		return this._focusedSession;
 	}
 
-	get focusedThread(): IThread {
+	get focusedThread(): IThread | undefined {
 		return this._focusedThread;
 	}
 
-	get focusedStackFrame(): IStackFrame {
+	get focusedStackFrame(): IStackFrame | undefined {
 		return this._focusedStackFrame;
 	}
 
-	setFocus(stackFrame: IStackFrame, thread: IThread, session: IDebugSession, explicit: boolean): void {
+	setFocus(stackFrame: IStackFrame | undefined, thread: IThread | undefined, session: IDebugSession | undefined, explicit: boolean): void {
 		const shouldEmitForStackFrame = this._focusedStackFrame !== stackFrame;
 		const shouldEmitForSession = this._focusedSession !== session;
 
@@ -72,29 +72,29 @@ export class ViewModel implements IViewModel {
 		return this._onDidFocusSession.event;
 	}
 
-	get onDidFocusStackFrame(): Event<{ stackFrame: IStackFrame, explicit: boolean }> {
+	get onDidFocusStackFrame(): Event<{ stackFrame: IStackFrame | undefined, explicit: boolean }> {
 		return this._onDidFocusStackFrame.event;
 	}
 
-	getSelectedExpression(): IExpression {
+	getSelectedExpression(): IExpression | undefined {
 		return this.selectedExpression;
 	}
 
-	setSelectedExpression(expression: IExpression) {
+	setSelectedExpression(expression: IExpression | undefined) {
 		this.selectedExpression = expression;
 		this.expressionSelectedContextKey.set(!!expression);
 		this._onDidSelectExpression.fire(expression);
 	}
 
-	get onDidSelectExpression(): Event<IExpression> {
+	get onDidSelectExpression(): Event<IExpression | undefined> {
 		return this._onDidSelectExpression.event;
 	}
 
-	getSelectedFunctionBreakpoint(): IFunctionBreakpoint {
+	getSelectedFunctionBreakpoint(): IFunctionBreakpoint | undefined {
 		return this.selectedFunctionBreakpoint;
 	}
 
-	setSelectedFunctionBreakpoint(functionBreakpoint: IFunctionBreakpoint): void {
+	setSelectedFunctionBreakpoint(functionBreakpoint: IFunctionBreakpoint | undefined): void {
 		this.selectedFunctionBreakpoint = functionBreakpoint;
 		this.breakpointSelectedContextKey.set(!!functionBreakpoint);
 	}