To reproduce, open two wizards in parallel. Use one to enter the flow and start creating a wallet,
then physically unplug the hw device at the correct time, and use the other wizard to trigger a rescan.
The rescan will unpair the hw device, resulting in device_manager.client_by_id to return None when
continuing the flow on the first wizard.
fixes https://github.com/spesmilo/electrum/issues/8858