Fix for the terminal shutdown issue (#4180)

This patch prevents creation of a new terminal when handling websocket handshaking request. The default behavior of `terminado.NamedTermManager` is to automatically start a new terminal in response to a websocket handshake, which prevents a terminal from being properly shut down in JupyterLab as reported in this [issue](jupyterlab/jupyterlab#5061).

Author: gdlmx

notebook/terminal/handlers.py

@@ -31,6 +31,8 @@ def origin_check(self):
     def get(self, *args, **kwargs):
         if not self.get_current_user():
             raise web.HTTPError(403)
+        if not args[0] in self.term_manager.terminals:
+            raise web.HTTPError(404)
         return super(TermSocket, self).get(*args, **kwargs)
 
     def on_message(self, message):