Invoking jupyter notebook with a notebook filename works but causes an exception

[lumberbot-app]

Describe the bug

It is unclear to me if one can launch jupyter notebook with a notebook filename to directly open it.

jupyter notebook <file.ipynb>

happens to work. However it causes the following exception

Error loading server extension jupyterlab
    Traceback (most recent call last):
      File "/home/callegar/PythonEnvs/Py3.8-devel/lib/python3.8/site-packages/notebook/notebookapp.py", line 1991, in init_server_extensions
        func(self)
      File "/home/callegar/PythonEnvs/Py3.8-devel/lib/python3.8/site-packages/jupyterlab/serverextension.py", line 33, in load_jupyter_server_extension
        extension.initialize()
      File "/home/callegar/PythonEnvs/Py3.8-devel/lib/python3.8/site-packages/jupyterlab/labapp.py", line 732, in initialize
        super().initialize()
      File "/home/callegar/PythonEnvs/Py3.8-devel/lib/python3.8/site-packages/jupyter_server/extension/application.py", line 408, in initialize
        self._prepare_handlers()
      File "/home/callegar/PythonEnvs/Py3.8-devel/lib/python3.8/site-packages/jupyter_server/extension/application.py", line 285, in _prepare_handlers
        self.initialize_handlers()
      File "/home/callegar/PythonEnvs/Py3.8-devel/lib/python3.8/site-packages/jupyterlab/labapp.py", line 661, in initialize_handlers
        relpath = os.path.relpath(self.serverapp.file_to_run, self.serverapp.root_dir)
    AttributeError: 'NotebookApp' object has no attribute 'root_dir'

To Reproduce

1. Invoke jupyter notebook <file.ipynb>

Expected behavior
Either the jupyter notebook starts directly at the specified notebook file with no exception or an error message is provided early saying that passing a filename on the jupyter command line is unsupported (actually it has historically always worked).

Desktop (please complete the following information):

• OS: ubuntu 20.04
• Browser: either chrome or firefox

---

Originally opened as jupyter/notebook#5971 by @callegar, migration requested by @Zsailer

[Zsailer]

@callegar, can you try upgrading to JupyterLab 3.0.9 and jupyter_server 1.4.1.

This should have been fixed by #415 in jupyter_server and jupyterlab/jupyterlab#9848 in jupyterlab.

[blink1073]

Closing as answered, cheers.