For validating topics we are using this pattern

"[a-zA-Z0-9-_.~%]{1,900}"

That was good for GCM according to the docs https://developers.google.com/cloud-messaging/ios/api/gcm/interface_g_c_m_pub_sub

But on FCM docs the pattern has changed a little, now we should use "[a-zA-Z0-9-_.~%]+" instead
https://firebase.google.com/docs/cloud-messaging/android/topic-messaging#build_send_requests

Lines to be changed
https://github.com/aerogear/aerogear-android-push/blob/master/aerogear-android-push/src/main/java/org/jboss/aerogear/android/unifiedpush/fcm/UnifiedPushConfig.java#L40
https://github.com/aerogear/aerogear-android-push/blob/master/aerogear-android-push/src/main/java/org/jboss/aerogear/android/unifiedpush/fcm/UnifiedPushConfig.java#L309