Add missing null check in VoicemailPlaybackPresenter

Bug: 22171979
Change-Id: Ia26ba711b888de3c787167bf9af99fcf26a80b7a
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index 92bc089..4597d35 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -286,7 +286,9 @@
             mMediaPlayer = null;
         }
 
-        mActivity.getWindow().clearFlags(LayoutParams.FLAG_KEEP_SCREEN_ON);
+        if (mActivity != null) {
+            mActivity.getWindow().clearFlags(LayoutParams.FLAG_KEEP_SCREEN_ON);
+        }
         disableProximitySensor(false /* waitForFarState */);
     }