Don't crash when removing a radio button

We were hitting an assert in RenderCustomPaint that the callback wasn't null.

R=eseidel@chromium.org, jackson@chromium.org

Review URL: https://codereview.chromium.org/1162023010

sdk/lib/framework/rendering/box.dart

@@ -680,14 +680,20 @@ class RenderCustomPaint extends RenderProxyBox {
 
   CustomPaintCallback _callback;
   void set callback (CustomPaintCallback value) {
-    assert(value != null);
+    assert(value != null || !attached);
     if (_callback == value)
       return;
     _callback = value;
     markNeedsPaint();
   }
 
+  void attach() {
+    assert(_callback != null);
+    super.attach();
+  }
+
   void paint(RenderObjectDisplayList canvas) {
+    assert(_callback != null);
     _callback(canvas);
     super.paint(canvas);
   }