Fix strict null checks

src/vs/workbench/parts/terminal/common/terminalService.ts

@@ -68,7 +68,10 @@ export abstract class TerminalService implements ITerminalService {
 		this._terminalFocusContextKey = KEYBINDING_CONTEXT_TERMINAL_FOCUS.bindTo(this._contextKeyService);
 		this._findWidgetVisible = KEYBINDING_CONTEXT_TERMINAL_FIND_WIDGET_VISIBLE.bindTo(this._contextKeyService);
 		this.onTabDisposed(tab => this._removeTab(tab));
-		this.onActiveTabChanged(() => this._onActiveInstanceChanged.fire(this.getActiveInstance()));
+		this.onActiveTabChanged(() => {
+			const instance = this.getActiveInstance();
+			this._onActiveInstanceChanged.fire(instance ? instance : undefined);
+		});
 
 		this._handleContextKeys();
 	}