Guard against race condition when querying for active VI.

I have a hunch this will fix a NullPointerException thrown from the
VehicleService when calling VehicleManager.getActiveVehicleInterface
(see
https://bugsnag.com/openxc/openxc-android-app/errors/546bde13f32e0785726662a5?event_id=546bde134758e1c6050291ef#stacktrace).