Fix NPE in setSpeakerphoneOn

Null check mActivity

Bug:22301343
Change-Id: Ia020f2a8c74cf16786af9ab7132da4eb9b9d0401
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index 22a3a45..f262105 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -679,7 +679,9 @@
             }
         } else {
             enableProximitySensor();
-            mActivity.getWindow().clearFlags(LayoutParams.FLAG_KEEP_SCREEN_ON);
+            if (mActivity != null) {
+                mActivity.getWindow().clearFlags(LayoutParams.FLAG_KEEP_SCREEN_ON);
+            }
         }
     }