Clean up after AppLifecycleTests (#12273)

Author: xster

testing/scenario_app/ios/Scenarios/ScenariosTests/AppLifecycleTests.m

@@ -140,5 +140,13 @@ - (void)testLifecycleChannel {
   XCTAssertEqualObjects(
       lifecycleEvents, expectedStates,
       @"AppLifecycleState transitions while presenting a second time not as expected");
+
+  // Dismantle.
+  [engine.lifecycleChannel setMessageHandler:nil];
+  [flutterVC dismissViewControllerAnimated:NO completion:nil];
+  flutterVC = nil;
+  [engine setViewController:nil];
+  [rootVC dismissViewControllerAnimated:NO completion:nil];
+  rootVC = nil;
 }
 @end