Null check for corner case NPE.

Fixes #3441
// FREEBIE

src/org/thoughtcrime/securesms/notifications/MessageNotifier.java

@@ -313,7 +313,7 @@ public class MessageNotifier {
         return;
       }
 
-      Uri uri = recipients.getRingtone();
+      Uri uri = recipients != null ? recipients.getRingtone() : null;
 
       if (uri == null) {
         String ringtone = TextSecurePreferences.getNotificationRingtone(context);