Use Instance trait for user_ns instead of Dict.

Instance(dict) is None by default, unlike Dict().

IPython/core/shellapp.py

@@ -201,7 +201,7 b' class InteractiveShellApp(Configurable):'
     )
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC')
     
-    user_ns = Dict(default_value=None)
+    user_ns = Instance(dict, args=None, allow_none=True)
     def _user_ns_changed(self, name, old, new):
         if self.shell is not None:
             self.shell.user_ns = new

IPython/kernel/zmq/ipkernel.py

@@ -87,7 +87,7 b' class Kernel(Configurable):'
         if self.shell is not None:
             self.shell.user_module = new
     
-    user_ns = Dict(default_value=None)
+    user_ns = Instance(dict, args=None, allow_none=True)
     def _user_ns_changed(self, name, old, new):
         if self.shell is not None:
             self.shell.user_ns = new