Fix the crash when the drawer is hidden.

sky/sdk/lib/framework/fn2.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 library fn;
 
 import 'app.dart';
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
@@ skipping 299 matching lines @@
     }
     if (parentData != null) {
       assert(root.parentData != null);
       root.parentData.merge(parentData); // this will throw if the types aren't approriate
       assert(parent != null);
       assert(parent.root != null);
       parent.root.markNeedsLayout();
     }
   }
 
-  void removeChild(UINode node) {
-    root.remove(node.root);
-    super.removeChild(node);
-  }
-
   void _remove() {
     assert(root != null);
     _nodeMap.remove(root);
     super._remove();
   }
 }
 
 abstract class OneChildRenderNodeWrapper extends RenderNodeWrapper {
   final UINode child;
 
   OneChildRenderNodeWrapper({ this.child, Object key }) : super(key: key);
 
-  void insert(RenderNodeWrapper child, dynamic slot) {
-    assert(slot == null);
-    root.child = child.root;
-  }
-
   void syncRenderNode(RenderNodeWrapper old) {
     super.syncRenderNode(old);
     UINode oldChild = old == null ? null : (old as OneChildRenderNodeWrapper).child;
     syncChild(child, oldChild, null);
   }
 
+  void insert(RenderNodeWrapper child, dynamic slot) {
+    assert(slot == null);
+    root.child = child.root;
+  }
+
+  void removeChild(UINode node) {
+    root.child = null;
+    super.removeChild(node);
+  }
+
   void _remove() {
-    assert(child != null);
-    removeChild(child);
+    if (child != null)
+      removeChild(child);
     super._remove();
   }
 }
 
 class Padding extends OneChildRenderNodeWrapper {
   RenderPadding root;
   final EdgeDims padding;
 
   Padding({ this.padding, UINode child, Object key })
     : super(child: child, key: key);
@@ skipping 68 matching lines @@
     key: key
   ) {
     assert(!_debugHasDuplicateIds());
   }
 
   void insert(RenderNodeWrapper child, dynamic slot) {
     assert(slot == null || slot is RenderNode);
     root.add(child.root, before: slot);
   }
 
+  void removeChild(UINode node) {
+    assert(root is ContainerRenderNodeMixin);
+    root.remove(node.root);
+    super.removeChild(node);
+  }
+
   void _remove() {
     assert(children != null);
     for (var child in children) {
       assert(child != null);
       removeChild(child);
     }
     super._remove();
   }
 
   bool _debugHasDuplicateIds() {
@@ skipping 499 matching lines @@
     assert(root.parent is RenderView);
   }
 }
 
 class Text extends Component {
   Text(this.data) : super(key: '*text*');
   final String data;
   bool get interchangeable => true;
   UINode build() => new Paragraph(text: data);
 }