Fixed crash of Observatory class-view element

runtime/observatory/lib/src/elements/class_view.dart

Index: runtime/observatory/lib/src/elements/class_view.dart
diff --git a/runtime/observatory/lib/src/elements/class_view.dart b/runtime/observatory/lib/src/elements/class_view.dart
index b16d68216562d47da384ca94441ff4485d6e0f04..362aa9a8c797dcc2fdf1d14494bb3671e6a50f54 100644
--- a/runtime/observatory/lib/src/elements/class_view.dart
+++ b/runtime/observatory/lib/src/elements/class_view.dart
@@ -419,10 +419,12 @@ class ClassViewElement extends HtmlElement implements Renderable {
                                                     queue: _r.queue)
                               ],
                             new DivElement()..classes = ['memberValue']
-                              ..children = [
-                                anyRef(_isolate, f.staticValue, _instances,
-                                       queue: _r.queue)
-                              ]
+                              ..children = f.staticValue == null
+                                ? const []
+                                : [
+                                     anyRef(_isolate, f.staticValue, _instances,
+                                            queue: _r.queue)
+                                ]
                           ]
                       ).toList())
                   ]