Container assumes the children attribute is not None

Thanks to @SylvainCorlay for pointing this out.

IPython/html/widgets/widget_container.py

@@ -1,34 +1,34 b''
 """ContainerWidget class.  
 
 Represents a container that can be used to group other widgets.
 """
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 from .widget import DOMWidget
 from IPython.utils.traitlets import Unicode, Tuple, TraitError
 
 class ContainerWidget(DOMWidget):
     _view_name = Unicode('ContainerView', sync=True)
 
     # 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)
 
     def _fire_children_displayed(self):
         for child in self.children:
             child._handle_displayed()
 
 
 class PopupWidget(ContainerWidget):
     _view_name = Unicode('PopupView', sync=True)
     
     description = Unicode(sync=True)
     button_text = Unicode(sync=True)