Add null check before using mCallback

There's some strange cases where onErrorInternal is being invoked
but mCallback is null. However, currently we only have a stack trace.
If this really is a problem, a bugreport will be needed. We can only
fix the NPE for now.

Fixes: 166187582
Test: Builds
Change-Id: I8ea932aadb15757383f273b0f6232b379c6b9ae1