`AttributeError: 'NoneType' object has no attribute 'start'`

[krassowski]

On main branch the installation of kernels ends up with an error for me:

1. Run: python -m ipykernel install --name mykernel --user
2. See:

Traceback (most recent call last):
  File "/home/krassowski/ipykernel/ipykernel/kernelapp.py", line 750, in main
    tg.start_soon(self.kernel.start)
                  ^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'start'

During handling of the above exception, another exception occurred:

  + Exception Group Traceback (most recent call last):
  |   File "<frozen runpy>", line 198, in _run_module_as_main
  |   File "<frozen runpy>", line 88, in _run_code
  |   File "/home/krassowski/ipykernel/ipykernel/__main__.py", line 5, in <module>
  |     app.launch_new_instance()
  |   File "/home/krassowski/.pyenv/versions/ipykernel/lib/python3.11/site-packages/traitlets/config/application.py", line 1077, in launch_instance
  |     app.start()
  |   File "/home/krassowski/ipykernel/ipykernel/kernelapp.py", line 740, in start
  |     run(self.main, backend=backend)
  |   File "/home/krassowski/.pyenv/versions/ipykernel/lib/python3.11/site-packages/anyio/_core/_eventloop.py", line 73, in run
  |     return async_backend.run(func, args, {}, backend_options)
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |   File "/home/krassowski/.pyenv/versions/ipykernel/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 1991, in run
  |     return runner.run(wrapper())
  |            ^^^^^^^^^^^^^^^^^^^^^
  |   File "/home/krassowski/.pyenv/versions/3.11.4/lib/python3.11/asyncio/runners.py", line 118, in run
  |     return self._loop.run_until_complete(task)
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |   File "/home/krassowski/.pyenv/versions/3.11.4/lib/python3.11/asyncio/base_events.py", line 653, in run_until_complete
  |     return future.result()
  |            ^^^^^^^^^^^^^^^
  |   File "/home/krassowski/.pyenv/versions/ipykernel/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 1979, in wrapper
  |     return await func(*args)
  |            ^^^^^^^^^^^^^^^^^
  |   File "/home/krassowski/ipykernel/ipykernel/kernelapp.py", line 748, in main
  |     async with create_task_group() as tg:
  |   File "/home/krassowski/.pyenv/versions/ipykernel/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 678, in __aexit__
  |     raise BaseExceptionGroup(
  | ExceptionGroup: unhandled errors in a TaskGroup (2 sub-exceptions)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/home/krassowski/ipykernel/ipykernel/kernelapp.py", line 750, in main
    |     tg.start_soon(self.kernel.start)
    |                   ^^^^^^^^^^^^^^^^^
    | AttributeError: 'NoneType' object has no attribute 'start'
    +---------------- 2 ----------------
    | Traceback (most recent call last):
    |   File "/home/krassowski/ipykernel/ipykernel/kernelapp.py", line 744, in _wait_to_enter_eventloop
    |     await self.kernel._eventloop_set.wait()
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^
    | AttributeError: 'NoneType' object has no attribute '_eventloop_set'
    +------------------------------------

The latter part seems related to #1292

[davidbrochart]

I opened #1310.