synchronize around getting last visibile uuid

src/main/java/eu/siacs/conversations/ui/ConversationFragment.java

@@ -1581,6 +1581,7 @@ public class ConversationFragment extends XmppFragment implements EditMessage.Ke
 		if (binding == null) {
 			return null;
 		}
+		synchronized (this.messageList) {
 			int pos = binding.messagesView.getLastVisiblePosition();
 			if (pos >= 0) {
 				Message message = null;
@@ -1597,6 +1598,7 @@ public class ConversationFragment extends XmppFragment implements EditMessage.Ke
 					return message.getUuid();
 				}
 			}
+		}
 		return null;
 	}