New tab causes crash for --existing kernels

[hrdl-github]

qtconsole 5.4.4
Reproduction:

$ jupyter-kernel &
[KernelApp] Starting kernel 'python3'
[KernelApp] Connection file: /home/user/.local/share/jupyter/runtime/kernel-267ae814-2df8-4ed5-8e9e-0906cf2446eb.json
[KernelApp] To connect a client: --existing kernel-267ae814-2df8-4ed5-8e9e-0906cf2446eb.json
$ jupyter-qtconsole --existing kernel-267ae814-2df8-4ed5-8e9e-0906cf2446eb.json
# Press C-t in GUI
Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/qtconsole/mainwindow.py", line 115, in create_tab_with_new_frontend
    widget = self.new_frontend_factory()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/qtconsole/qtconsoleapp.py", line 194, in new_frontend_master
    if self.kernel_manager.ipykernel:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'ipykernel'