diff --git a/examples/fitness/lib/feed.dart b/examples/fitness/lib/feed.dart
index 06943f92454..ac7d09f018b 100644
--- a/examples/fitness/lib/feed.dart
+++ b/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,
diff --git a/examples/fitness/lib/main.dart b/examples/fitness/lib/main.dart
index e269113cb88..49e92356c1a 100644
--- a/examples/fitness/lib/main.dart
+++ b/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();
diff --git a/packages/flutter/lib/widgets/framework.dart b/packages/flutter/lib/widgets/framework.dart
index 424bfaa30ee..a234d1c35ec 100644
--- a/packages/flutter/lib/widgets/framework.dart
+++ b/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);
-    assert(child.parent == this);
-    assert(child.renderObject != null);
-    _renderObject = child.renderObject;
-    assert(_renderObject == renderObject); // in case a subclass reintroduces it
+    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) {