Fix the crash when the drawer is hidden.

sky/sdk/lib/framework/fn2.dart

Index: sky/sdk/lib/framework/fn2.dart
diff --git a/sky/sdk/lib/framework/fn2.dart b/sky/sdk/lib/framework/fn2.dart
index 77fde9afa1620bbc00a1f983ab1d583d09d34f08..a3adb268742274348a5a80bfed9a3dc4e059476b 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -317,11 +317,6 @@ abstract class RenderNodeWrapper extends UINode {
     }
   }
 
-  void removeChild(UINode node) {
-    root.remove(node.root);
-    super.removeChild(node);
-  }
-
   void _remove() {
     assert(root != null);
     _nodeMap.remove(root);
@@ -334,20 +329,25 @@ abstract class OneChildRenderNodeWrapper extends RenderNodeWrapper {
 
   OneChildRenderNodeWrapper({ this.child, Object key }) : super(key: key);
 
+  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 syncRenderNode(RenderNodeWrapper old) {
-    super.syncRenderNode(old);
-    UINode oldChild = old == null ? null : (old as OneChildRenderNodeWrapper).child;
-    syncChild(child, oldChild, null);
+  void removeChild(UINode node) {
+    root.child = null;
+    super.removeChild(node);
   }
 
   void _remove() {
-    assert(child != null);
-    removeChild(child);
+    if (child != null)
+      removeChild(child);
     super._remove();
   }
 }
@@ -436,6 +436,12 @@ abstract class OneChildListRenderNodeWrapper extends RenderNodeWrapper {
     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) {