ensure control messages are handled without optional control_thread

and start handling them in `.start()` instead of `__init__`

Author: minrk

ipykernel/kernelbase.py

@@ -205,8 +205,6 @@ def __init__(self, **kwargs):
             self.control_handlers[msg_type] = getattr(self, msg_type)
 
         self.control_queue = Queue()
-        if 'control_thread' in kwargs:
-            kwargs['control_thread'].io_loop.add_callback(self.poll_control_queue)
 
     @gen.coroutine
     def dispatch_control(self, msg):
@@ -452,6 +450,13 @@ def start(self):
 
         self.control_stream.on_recv(self.dispatch_control, copy=False)
 
+        if self.control_thread:
+            control_loop = self.control_thread.io_loop
+        else:
+            control_loop = self.io_loop
+
+        control_loop.add_callback(self.poll_control_queue)
+
         self.shell_stream.on_recv(
             partial(
                 self.schedule_dispatch,