diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 80ed914f68081..162686d769d00 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -21,7 +21,7 @@
 		"./vs/base/common/diff/diff.ts",
 		"./vs/base/common/diff/diffChange.ts",
 		"./vs/base/common/errors.ts",
-		// "./vs/base/common/event.ts",
+		"./vs/base/common/event.ts",
 		"./vs/base/common/functional.ts",
 		"./vs/base/common/idGenerator.ts",
 		"./vs/base/common/iterator.ts",
diff --git a/src/vs/base/common/event.ts b/src/vs/base/common/event.ts
index 431960a43f2e6..0447231832da4 100644
--- a/src/vs/base/common/event.ts
+++ b/src/vs/base/common/event.ts
@@ -56,13 +56,16 @@ export class Emitter<T> {
 
 	private static readonly _noop = function () { };
 
-	private _event: Event<T>;
+	private _event: Event<T> | null;
 	private _disposed: boolean;
-	private _deliveryQueue: [Listener, T][];
-	protected _listeners: LinkedList<Listener>;
-
-	constructor(private _options?: EmitterOptions) {
-
+	private _deliveryQueue: [Listener, (T | undefined)][] | null;
+	protected _listeners: LinkedList<Listener> | null;
+
+	constructor(private _options: EmitterOptions | null = null) {
+		this._event = null;
+		this._disposed = false;
+		this._deliveryQueue = null;
+		this._listeners = null;
 	}
 
 	/**
@@ -98,8 +101,11 @@ export class Emitter<T> {
 						result.dispose = Emitter._noop;
 						if (!this._disposed) {
 							remove();
-							if (this._options && this._options.onLastListenerRemove && this._listeners.isEmpty()) {
-								this._options.onLastListenerRemove(this);
+							if (this._options && this._options.onLastListenerRemove) {
+								const hasListeners = (this._listeners && !this._listeners.isEmpty());
+								if (!hasListeners) {
+									this._options.onLastListenerRemove(this);
+								}
 							}
 						}
 					}
@@ -133,7 +139,7 @@ export class Emitter<T> {
 			}
 
 			while (this._deliveryQueue.length > 0) {
-				const [listener, event] = this._deliveryQueue.shift();
+				const [listener, event] = this._deliveryQueue.shift()!;
 				try {
 					if (typeof listener === 'function') {
 						listener.call(undefined, event);
@@ -149,7 +155,7 @@ export class Emitter<T> {
 
 	dispose() {
 		if (this._listeners) {
-			this._listeners = undefined;
+			this._listeners = null;
 		}
 		if (this._deliveryQueue) {
 			this._deliveryQueue.length = 0;
@@ -184,7 +190,7 @@ export class AsyncEmitter<T extends IWaitUntil> extends Emitter<T> {
 		}
 
 		while (this._asyncDeliveryQueue.length > 0) {
-			const [listener, event, thenables] = this._asyncDeliveryQueue.shift();
+			const [listener, event, thenables] = this._asyncDeliveryQueue.shift()!;
 			try {
 				if (typeof listener === 'function') {
 					listener.call(undefined, event);
@@ -208,7 +214,7 @@ export class EventMultiplexer<T> implements IDisposable {
 
 	private readonly emitter: Emitter<T>;
 	private hasListeners = false;
-	private events: { event: Event<T>; listener: IDisposable; }[] = [];
+	private events: { event: Event<T>; listener: IDisposable | null; }[] = [];
 
 	constructor() {
 		this.emitter = new Emitter<T>({
@@ -251,12 +257,14 @@ export class EventMultiplexer<T> implements IDisposable {
 		this.events.forEach(e => this.unhook(e));
 	}
 
-	private hook(e: { event: Event<T>; listener: IDisposable; }): void {
+	private hook(e: { event: Event<T>; listener: IDisposable | null; }): void {
 		e.listener = e.event(r => this.emitter.fire(r));
 	}
 
-	private unhook(e: { event: Event<T>; listener: IDisposable; }): void {
-		e.listener.dispose();
+	private unhook(e: { event: Event<T>; listener: IDisposable | null; }): void {
+		if (e.listener) {
+			e.listener.dispose();
+		}
 		e.listener = null;
 	}
 
@@ -270,7 +278,7 @@ export function fromPromise<T =any>(promise: Thenable<T>): Event<T> {
 	let shouldEmit = false;
 
 	promise
-		.then(null, () => null)
+		.then(undefined, () => null)
 		.then(() => {
 			if (!shouldEmit) {
 				setTimeout(() => emitter.fire(), 0);
@@ -321,11 +329,11 @@ export function anyEvent<T>(...events: Event<T>[]): Event<T> {
 }
 
 export function debounceEvent<T>(event: Event<T>, merger: (last: T, event: T) => T, delay?: number, leading?: boolean): Event<T>;
-export function debounceEvent<I, O>(event: Event<I>, merger: (last: O, event: I) => O, delay?: number, leading?: boolean): Event<O>;
-export function debounceEvent<I, O>(event: Event<I>, merger: (last: O, event: I) => O, delay: number = 100, leading = false): Event<O> {
+export function debounceEvent<I, O>(event: Event<I>, merger: (last: O | undefined, event: I) => O, delay?: number, leading?: boolean): Event<O>;
+export function debounceEvent<I, O>(event: Event<I>, merger: (last: O | undefined, event: I) => O, delay: number = 100, leading = false): Event<O> {
 
 	let subscription: IDisposable;
-	let output: O = undefined;
+	let output: O | undefined = undefined;
 	let handle: any = undefined;
 	let numDebouncedCalls = 0;
 
@@ -497,10 +505,10 @@ export function stopwatch<T>(event: Event<T>): Event<number> {
  * // 4
  * ```
  */
-export function buffer<T>(event: Event<T>, nextTick = false, buffer: T[] = []): Event<T> {
-	buffer = buffer.slice();
+export function buffer<T>(event: Event<T>, nextTick = false, _buffer: T[] = []): Event<T> {
+	let buffer: T[] | null = _buffer.slice();
 
-	let listener = event(e => {
+	let listener: IDisposable | null = event(e => {
 		if (buffer) {
 			buffer.push(e);
 		} else {
@@ -509,7 +517,9 @@ export function buffer<T>(event: Event<T>, nextTick = false, buffer: T[] = []):
 	});
 
 	const flush = () => {
-		buffer.forEach(e => emitter.fire(e));
+		if (buffer) {
+			buffer.forEach(e => emitter.fire(e));
+		}
 		buffer = null;
 	};
 
@@ -531,7 +541,9 @@ export function buffer<T>(event: Event<T>, nextTick = false, buffer: T[] = []):
 		},
 
 		onLastListenerRemove() {
-			listener.dispose();
+			if (listener) {
+				listener.dispose();
+			}
 			listener = null;
 		}
 	});