quick input - fix integration tests

src/vs/base/parts/quickinput/browser/quickInput.ts

@@ -724,7 +724,7 @@ class QuickPick<T extends IQuickPickItem> extends QuickInput implements IQuickPi
 				this._selectedItems = selectedItems as T[];
 				this.onDidChangeSelectionEmitter.fire(selectedItems as T[]);
 				if (selectedItems.length) {
-					this.onDidAcceptEmitter.fire({ inBackground: (event as MouseEvent).button === 1 /* mouse middle click */ });
+					this.onDidAcceptEmitter.fire({ inBackground: event instanceof MouseEvent && event.button === 1 /* mouse middle click */ });
 				}
 			}));
 			this.visibleDisposables.add(this.ui.list.onChangedCheckedElements(checkedItems => {