regression from cea4238b81
```
975.04 | E | plugin.DeviceMgr | failed to create client for digitalbitbox at <REDACTED>: AttributeError("'NoneType' object has no attribute 'get_passphrase'")
Traceback (most recent call last):
File "...\electrum\electrum\plugin.py", line 620, in list_pairable_device_infos
soft_device_id = client.get_soft_device_id()
File "...\electrum\electrum\plugins\hw_wallet\plugin.py", line 251, in get_soft_device_id
root_fp = self.request_root_fingerprint_from_device()
File "...\electrum\electrum\plugin.py", line 362, in wrapper
return run_in_hwd_thread(partial(func, *args, **kwargs))
File "...\electrum\electrum\plugin.py", line 352, in run_in_hwd_thread
return func()
File "...\electrum\electrum\plugins\hw_wallet\plugin.py", line 264, in request_root_fingerprint_from_device
child_of_root_xpub = self.get_xpub("m/0'", xtype='standard')
File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 115, in get_xpub
reply = self._get_xpub(bip32_path)
File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 110, in _get_xpub
if self.check_device_dialog():
File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 197, in check_device_dialog
if not self.password_dialog(msg):
File "...\electrum\electrum\plugins\digitalbitbox\digitalbitbox.py", line 159, in password_dialog
password = self.handler.get_passphrase(msg, False)
AttributeError: 'NoneType' object has no attribute 'get_passphrase'
```
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