Fix to remove potential memory leak on Jupyter Server ZMQChannelHandl…

…er code (#682)

Co-authored-by: Steven Silvester <steven.silvester@ieee.org>

jupyter_server/services/kernels/handlers.py

@@ -387,7 +387,10 @@ async def _register_session(self):
         if stale_handler:
             self.log.warning("Replacing stale connection: %s", self.session_key)
             await stale_handler.close()
-        self._open_sessions[self.session_key] = self
+        if (
+            self.kernel_id in self.kernel_manager
+        ):  # only update open sessions if kernel is actively managed
+            self._open_sessions[self.session_key] = self
 
     def open(self, kernel_id):
         super(ZMQChannelsHandler, self).open()