Fix NPE when plugging and unplugging earphones.

Crashes because the VoicemailPlaybackPresenter is alerted when the state
of the headset changes but the corresponding view may be out of focus.

Bug: 25090589
Change-Id: Iabe24eb969777ae3919ede05474f35d6c596b20e
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index 9b4e20d..c0884ad 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -749,6 +749,10 @@
      * the presenter without the presenter triggering the audio manager and duplicating actions.
      */
     public void setSpeakerphoneOn(boolean on) {
+        if (mView == null) {
+            return;
+        }
+
         mView.onSpeakerphoneOn(on);
 
         mIsSpeakerphoneOn = on;