Merge pull request #8107 from SylvainCorlay/instance_type_allow_none

allow_none=False by default for Instance and Type

IPython/html/notebookapp.py

@@ -665,7 +665,7 @@ class NotebookApp(BaseIPythonApplication):
         help='The config manager class to use'
     )
 
-    kernel_spec_manager = Instance(KernelSpecManager)
+    kernel_spec_manager = Instance(KernelSpecManager, allow_none=True)
 
     kernel_spec_manager_class = Type(
         default_value=KernelSpecManager,

IPython/html/widgets/widget.py

@@ -129,7 +129,7 @@ class Widget(LoggingConfigurable):
         If empty, look in the global registry.""", sync=True)
     _view_name = Unicode(None, allow_none=True, help="""Default view registered in the front-end
         to use to represent the widget.""", sync=True)
-    comm = Instance('IPython.kernel.comm.Comm')
+    comm = Instance('IPython.kernel.comm.Comm', allow_none=True)
     
     msg_throttle = Int(3, sync=True, help="""Maximum number of msgs the 
         front-end can send before receiving an idle msg from the back-end.""")