bitbox02: AttributeError: 'NoneType' object has no attribute 'electrum_encryption_key'

[Nicolas Kuttler]

  File "/path/to/venv/lib/python3.11/site-packages/electrum/plugins/bitbox02/bitbox02.py", line 207, 
in get_password_for_storage_encryption
    xpub = self.bitbox02_device.electrum_encryption_key(derivation_list)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'electrum_encryption_key'

The issue seems easy enough to fix, I will make a pull request in a few minutes, but I'm not sure if the fix is "proper".