Fix to add session only after kernel validation

Author: Vishwajeet0510

notebook/services/kernels/handlers.py

@@ -219,6 +219,7 @@ def pre_get(self):
         # We don't want to wait forever, because browsers don't take it well when
         # servers never respond to websocket connection requests.
         kernel = self.kernel_manager.get_kernel(self.kernel_id)
+        self._open_sessions[self.session_key] = self
         self.session.key = kernel.session.key
         future = self.request_kernel_info()
 
@@ -251,7 +252,6 @@ def _register_session(self):
         if stale_handler:
             self.log.warning("Replacing stale connection: %s", self.session_key)
             yield stale_handler.close()
-        self._open_sessions[self.session_key] = self
 
     def open(self, kernel_id):
         super(ZMQChannelsHandler, self).open()