Update recipients on UI thread.

Fixes #3402
// FREEBIE

src/org/thoughtcrime/securesms/ConversationActivity.java

@@ -797,10 +797,15 @@ public class ConversationActivity extends PassphraseRequiredActionBarActivity
   }
 
   @Override
-  public void onModified(Recipients recipients) {
+  public void onModified(final Recipients recipients) {
+    titleView.post(new Runnable() {
+      @Override
+      public void run() {
         titleView.setTitle(recipients);
         setBlockedUserState(recipients);
       }
+    });
+  }
 
   private void initializeReceivers() {
     securityUpdateReceiver = new BroadcastReceiver() {