Container assumes the children attribute is not None

Thanks to @SylvainCorlay for pointing this out.

IPython/html/widgets/widget_container.py

@@ -15,9 +15,9 @@ class ContainerWidget(DOMWidget):
     # Child widgets in the container.
     # Using a tuple here to force reassignment to update the list.
     # When a proper notifying-list trait exists, that is what should be used here.
-    children = Tuple(sync=True)
+    children = Tuple(sync=True, allow_none=False)
 
-    def __init__(self, children = None, **kwargs):
+    def __init__(self, children = (), **kwargs):
         kwargs['children'] = children
         super(ContainerWidget, self).__init__(**kwargs)
         self.on_displayed(ContainerWidget._fire_children_displayed)