fix npe

src/vs/base/common/event.ts

@@ -152,11 +152,11 @@ export function fromEventEmitter<T>(emitter: EventEmitter, eventType: string): E
 }
 
 export function mapEvent<I,O>(event: Event<I>, map: (i:I)=>O): Event<O> {
-	return (listener, thisArgs?, disposables?) => event(i => listener(map(i)), thisArgs, disposables);
+	return (listener, thisArgs = null, disposables?) => event(i => listener.call(thisArgs, map(i)), null, disposables);
 }
 
 export function filterEvent<T>(event: Event<T>, filter: (e:T)=>boolean): Event<T> {
-	return (listener, thisArgs?, disposables?) => event(e => filter(e) && listener(e), thisArgs, disposables);
+	return (listener, thisArgs = null, disposables?) => event(e => filter(e) && listener.call(thisArgs, e), null, disposables);
 }
 
 enum EventDelayerState {