Fix nullptr access in mixer group. This was only ever an issue on booboot and not in flight.
Fix nullptr access in mixer group. This was only ever an issue on booboot and not in flight.

PX4/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,krbeverx/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,PX4/Firmware,Aerotenna/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,acfloria/Firmware,dagar/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,krbeverx/Firmware,jlecoeur/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,dagar/Firmware,jlecoeur/Firmware,dagar/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,jlecoeur/Firmware