Fix NullPointException BatterySaverModeTest

Bug: 24193083

Change-Id: I998f60a213313b130d1b48e1b25739c3fbc5739c
diff --git a/tests/tests/voicesettings/service/src/android/voicesettings/service/MainInteractionSession.java b/tests/tests/voicesettings/service/src/android/voicesettings/service/MainInteractionSession.java
index c2b7e18..aff1160 100644
--- a/tests/tests/voicesettings/service/src/android/voicesettings/service/MainInteractionSession.java
+++ b/tests/tests/voicesettings/service/src/android/voicesettings/service/MainInteractionSession.java
@@ -24,6 +24,7 @@
 import static android.provider.Settings.ACTION_VOICE_CONTROL_BATTERY_SAVER_MODE;
 import static android.provider.Settings.EXTRA_BATTERY_SAVER_MODE_ENABLED;
 
+import android.app.VoiceInteractor;
 import android.content.Context;
 import android.content.Intent;
 import android.os.AsyncTask;
@@ -138,9 +139,10 @@
 
     @Override
     public void onRequestCompleteVoice(CompleteVoiceRequest request) {
-        CharSequence prompt = request.getVoicePrompt().getVoicePromptAt(0);
+        VoiceInteractor.Prompt prompt = request.getVoicePrompt();
+        CharSequence message = (prompt != null ? prompt.getVoicePromptAt(0) : "(none)");
         Log.i(TAG, "in Session testcasetype = " + mTestType +
-                ", onRequestCompleteVoice recvd. message = " + prompt);
+                ", onRequestCompleteVoice recvd. message = " + message);
         AsyncTaskArg asyncTaskArg = new AsyncTaskArg().setRequest(request).setCompleteReq(true);
         newTask().execute(asyncTaskArg);
     }