Hack around Fitness.app crashing if it can't load data

I'm not sure this is the correct approach, but it
at least mitigates https://github.com/domokit/sky_engine/issues/804
for now.

I also made it not crash if you haven't set a goalWeight.

@collinjackson

examples/fitness/lib/feed.dart

@@ -194,7 +194,7 @@ class FeedFragment extends StatefulComponent {
           dataSet.add(new Point(x, y));
       }
     }
-    if (userData.goalWeight > 0.0) {
+    if (userData.goalWeight != null && userData.goalWeight > 0.0) {
       startY = math.min(startY, userData.goalWeight);
       endY = math.max(endY, userData.goalWeight);
     }

examples/fitness/lib/main.dart

@@ -92,7 +92,10 @@ class FitnessApp extends App {
     super.didMount();
     loadFitnessData().then((UserData data) {
       setState(() => _userData = data);
-    }).catchError((e) => print("Failed to load data: $e"));
+    }).catchError((e) {
+      print("Failed to load data: $e");
+      setState(() => _userData = new UserDataImpl());
+    });
   }
 
   void initState() {