Fix potential NPE in VoicemailPlaybackPresenter

Bug: 26092525
Change-Id: Id3e54a197ed8bfb51b9b01bbc98a4ee81b28d9ee
diff --git a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
index 8191d1d..93ff002 100644
--- a/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
+++ b/src/com/android/dialer/voicemail/VoicemailPlaybackPresenter.java
@@ -386,7 +386,7 @@
             public void onPostExecute(Boolean hasContent) {
                 if (hasContent) {
                     prepareContent();
-                } else {
+                } else if (mView != null) {
                     mView.resetSeekBar();
                     mView.setClipPosition(0, mDuration.get());
                 }