Can't unselect all internal device sensors, even if an external device is connected

[manuelzomer]

Describe the bug
In the sensor settings it is not possible to unselect all internal sensors (one sensor always stays) even if an external device is connected. In comparison, the Android version lets you unselect all internal sensors as long as any external sensor of a connected device is selected.

To Reproduce
Steps to reproduce the behavior:

1. Go to the sensor settings
2. Add an external device
3. Select a sensor of the external device
4. Try to unselect all internal sensors

Expected behavior
As long as an external device is connected (and a sensor of this connected device is selected), it should be possible to unselect all internal sensors.

[emiliopavia]

@manuelzomer @VKin-Arduino we inherited this behavior from the original codebase. The given is that we cannot have the sensors tab in the experiment empty. If we want to change the current behavior there's one edge case to consider:

• the user selects an external sensor;
• the user deselects all the internal sensors;
• the user unpairs the BLE device.

At this point there's no selected sensor in settings. What should happen then?

[manuelzomer]

@emiliopavia On Android the behaviour for this edge case is, that as soon as the user unpairs the BLE device by clicking "Forget this device", the first internal sensor of the list get's selected automatically again. For consistency reasons it would be ideal to have that same behaviour for iOS as well.