Fix shutdown.

Author: T4rk1n

jupyter_dash/jupyter_app.py

@@ -220,6 +220,8 @@ def run_server(
         old_server = self._server_threads.get((host, port))
         if old_server:
             old_server.kill()
+            old_server.join()
+            del self._server_threads[(host, port)]
 
         # Configure pathname prefix
         requests_pathname_prefix = self.config.get('requests_pathname_prefix', None)