Fix crash when clicking an action in context menu for a removed device

Devices can be added and removed while the context menu is open.
Sfter an action has clicked that needs to access a device, we therefore
have to check if its position in the model has changed, and more
importantly, if it is still there at all in order to prevent a crash.

BUG: 315298
FIXED-IN: 4.10.1
REVIEW: 108989