Add engine non-nil asserts

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -1311,7 +1311,7 @@ - (void)createTouchRateCorrectionVSyncClientIfNeeded {
     return;
   }
 
-  // XXX: Errr, shouldn't we be ensuring self.engine isn't nil here?
+  NSAssert(self.engine, @"engine must not be nil");
   flutter::Shell& shell = self.engine.shell;
   auto callback = [](std::unique_ptr<flutter::FrameTimingsRecorder> recorder) {
     // Do nothing in this block. Just trigger system to callback touch events with correct rate.
@@ -1396,7 +1396,7 @@ - (void)viewDidLayoutSubviews {
   if (firstViewBoundsUpdate && applicationOrSceneIsActive && self.engine) {
     [self surfaceUpdated:YES];
 
-    // XXX: Check self.engine isn't nil...
+    NSAssert(self.engine, @"engine must not be nil");
     flutter::Shell& shell = self.engine.shell;
     fml::TimeDelta waitTime =
 #if FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG