Don’t flash a message while fitness data is loading

examples/fitness/lib/feed.dart

@@ -214,6 +214,8 @@ class FeedFragment extends StatefulComponent {
 
   Widget buildBody() {
     TextStyle style = Theme.of(this).text.title;
+    if (userData == null)
+      return new Material(type: MaterialType.canvas);
     if (userData.items.length == 0)
       return new Material(
         type: MaterialType.canvas,

examples/fitness/lib/main.dart

@@ -86,7 +86,7 @@ class UserDataImpl extends UserData {
 
 class FitnessApp extends App {
   NavigationState _navigationState;
-  UserDataImpl _userData = new UserDataImpl();
+  UserDataImpl _userData;
 
   void didMount() {
     super.didMount();

packages/flutter/lib/widgets/framework.dart

@@ -425,10 +425,14 @@ abstract class TagNode extends Widget {
   void _sync(Widget old, dynamic slot) {
     Widget oldChild = old == null ? null : (old as TagNode).child;
     child = syncChild(child, oldChild, slot);
+    if (child != null) {
       assert(child.parent == this);
       assert(child.renderObject != null);
       _renderObject = child.renderObject;
       assert(_renderObject == renderObject); // in case a subclass reintroduces it
+    } else {
+      _renderObject = null;
+    }
   }
 
   void updateSlot(dynamic newSlot) {