Add this line electric_always_available=1 in the [electrical] section of the aircraft.cfg (if it is not already there).

[electrical]
max_battery_voltage=24
generator_alternator_voltage=28
max_generator_alternator_amps=60
electric_always_available=1
autopilot=0, 5, 17
avionics_bus=0, 10, 17
avionics=1, 5, 17
gear_warning=0, 2, 17
fuel_pump=0, 5, 17
fuel_transfer_pump = 0, 5, 17
light_nav=0, 5, 17
light_beacon=0, 5, 17
light_landing=0, 5, 17
light_taxi=0, 5, 17
light_strobe=0, 5, 17
light_panel=0, 5, 17

Having that there should prevent the loss of the avionics during the flight.