客户端在Android上等待来自服务器的消息

I'm just trying to do a connection from a server on Java and a client on android. my problem is that I made a thread for waiting a message to print it on a EditText. I only recive the first message that is sent from the server like a default message, saying "connection succesful" Now, when I send a second message the app just close and I have to force the close. I'm sure that the problem is on the thread that is where I have the method leermensajes(). I don't know what to do, now that I'm newbie on Android and also I don't have much experience programming.I'm just trying to do a connection from a serve