[Effen] fix _ensureDepth to work on root of tree, oops

sky/framework/fn.dart

Index: sky/framework/fn.dart
diff --git a/sky/framework/fn.dart b/sky/framework/fn.dart
index 20a8b1cffac8f26e8ddf0d4eb3b09f6a79a03205..e01c34ddbc4f1a48cc2068b56ca8e8a524139e47 100644
--- a/sky/framework/fn.dart
+++ b/sky/framework/fn.dart
@@ -94,8 +94,12 @@ abstract class UINode {
   int _nodeDepth;
   void _ensureDepth() {
     if (_nodeDepth == null) {
-      _parent.ensureDepth();
-      _nodeDepth = _parent._nodeDepth + 1;
+      if (_parent != null) {
+        _parent.ensureDepth();
+        _nodeDepth = _parent._nodeDepth + 1;
+      } else {
+        _nodeDepth = 0;
+      }
     }
   }