Check for null callback

The modelData callback may be nullified before actually being invoked,
e.g. when calling startRecognition() on an already running model, with
a different callback.

Fixes: 225768984
Test: Manual reproduction on the issue and then applying the fix and
      verifying that it works.
Change-Id: I089061123c8cf12eef19fdf16bb3a2c106ea2438

services/voiceinteraction/java/com/android/server/soundtrigger/SoundTriggerHelper.java

@@ -923,7 +923,10 @@ public class SoundTriggerHelper implements SoundTrigger.StatusListener {
         if (modelData != null && modelData.isModelStarted()) {
             modelData.setStoppedLocked();
             try {
-                modelData.getCallback().onRecognitionPaused();
+                IRecognitionStatusCallback callback = modelData.getCallback();
+                if (callback != null) {
+                    callback.onRecognitionPaused();
+                }
             } catch (DeadObjectException e) {
                 forceStopAndUnloadModelLocked(modelData, e);
             } catch (RemoteException e) {