Fix: KNX undefined error when updating settings

[wouter-athom]

When the interface cannot be found on the network, the app will show an error when trying to save the settings.

This can be reproduced through the following steps:

• Set up the app properly and add a device
• Run the app without the remote flag
• Try to change the settings
• See that the error is thrown
• Run the app with the remote flag and see that the changes to the settings were not applied

After this fix, no error message will be shown and settings will still get applied.

Asana: https://app.asana.com/0/1198303871831434/1204767921017882/f