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

services/core/java/com/android/server/biometrics/sensors/AcquisitionClient.java

@@ -98,7 +98,11 @@ public abstract class AcquisitionClient<T> extends ClientMonitor<T> implements I
         }
 
         if (finish) {
-            mCallback.onClientFinished(this, false /* success */);
+            if (mCallback == null) {
+                Slog.e(TAG, "Callback is null, perhaps the client hasn't been started yet?");
+            } else {
+                mCallback.onClientFinished(this, false /* success */);
+            }
         }
     }