This is Mikey with Airship Technical Support. I understand you're having trouble with GCM/FCM registration. Have you had a chance to take a look at out GCM/FCM troubleshooting guide? This is a good place to address invalid sender errors. You can also take a look at our Android troubleshooting guide for further assistance.
Something important to note: If you are calling takeoff using the AutoPilot class you should not receive any notification prompt in order to start receiving pushes.
Please let me know if you have any questions or need further assistance and I'll be happy to assist!
Airship | Technical Support Engineer | Portland, OR