Can fail to connect after switching from client mode to server mode

[rppicomidi]

To reproduce:

1. In client mode, connect to a BLE MIDI peripheral and save as the current preset.
2. Turn off the BLE MIDI peripheral and turn off the midi2piousbhub device.
3. Turn on the midi2piousbhub device. It will attempt to connect to the BLE MIDI peripheral, but it is off
4. Switch to server mode
5. Using an iPad BLE MIDI client or similar, attempt to connect to the midi2piousbhub.
6. Observe the connection starts to happen, but then fails.