Check if boxes still available when restoring state

Apparently when the device is idle long enough the system will clean
up the dialog view for the hotspot band preference. This will cause
restore state to fail once it decides to pause the activity. To avoid
this we should make sure the boxes are not null.

Test: robotest
Bug: 78445411
Change-Id: I2a82b01f894f62be6c5209c78faddc4089aa6301