Be more defensive when looking up signals in button event handler.
Be more defensive when looking up signals in button event handler.

openxc/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,ene-ilies/vi-firmware,ene-ilies/vi-firmware,openxc/vi-firmware,mgiannikouris/vi-firmware,openxc/vi-firmware,openxc/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,mgiannikouris/vi-firmware