bec error message if gui was closed manually

If bec gui was closed manually outside BECIPythonClient this error message is displayed during shutdown:

• demo [1/2] ❯❯ d2 = gui.add_dock()

• demo [2/2] ❯❯ bar = d2.add_widget_bec('SpiralProgressBar')

• demo [3/2] ❯❯ bar.set_value([10,20,30,40,50])
Truncated list of 'values' to match 1 bars.

# here gui closed manually

• demo [4/2] ❯❯ exit
Traceback (most recent call last):
  File "/Users/janwyzula/miniforge3/envs/bw-pyproject/bin/bec", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/janwyzula/PSI/bec/bec_ipython_client/bec_ipython_client/main.py", line 256, in main
    main_dict["bec"].shutdown()
  File "/Users/janwyzula/PSI/bec/bec_ipython_client/bec_ipython_client/main.py", line 127, in shutdown
    self.gui.close()
  File "/Users/janwyzula/PSI/bec_widgets/bec_widgets/cli/client_utils.py", line 125, in close
    self._run_rpc("close", (), wait_for_rpc_response=True)
  File "/Users/janwyzula/PSI/bec_widgets/bec_widgets/cli/client_utils.py", line 255, in _run_rpc
    if not response.content["accepted"]:
           ^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'content'

If you suspect this is an IPython 8.24.0 bug, please report it at:
    https://github.com/ipython/ipython/issues
or send an email to the mailing list at ipython-dev@python.org

You can print a more detailed traceback right now with "%tb", or use "%debug"
to interactively debug it.

Extra-detailed tracebacks for bug-reporting purposes can be enabled via:
    %config Application.verbose_crash=True