error on switching layout in android -

i'm trying switch layout error below keeps coming up.

java.lang.nullpointerexception: attempt invoke virtual method 'void android.view.viewgroup.setid(int)' on null object reference 

android studio complaining on "setcontentview(r.layout.activity_main);"

i'm not sure how fix error. googled error , tried several of fixes none worked thread networking added. i'm missing simple. appreciated.

protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     new thread(new runnable() {         public void run() {             try {                 new chatserver();             } catch (ioexception e) {                 e.printstacktrace();             }             setcontentview(r.layout.activity_main);             client = new client();             client.start();             network.register(client);             client.addlistener(new listener() {                 public void connected(connection connection) {                     network.registername registername = new network.registername();                     registername.name = name;                     client.sendtcp(registername);                 }                  public void received(connection connection, object object) {                     if (object instanceof network.chatmessage) {                         network.chatmessage chatmessage = (network.chatmessage) object;                         system.out.println(chatmessage.text);                         return;                     }                 }             });             host = client.discoverhosts(54777, 5000);             while (host == null) {                 host = client.discoverhosts(54777, 5000);                 system.out.println(host);             }             if (host.isempty()) {                 addmsg("no hosts available");                 system.exit(1);             }             final inetaddress address = host.get(0);             setcontentview(r.layout.promptname);         }     }).start();     } 

move setcontentview(r.layout.activity_main); outside anonymous-inner thread class, , rid of setcontentview(r.layout.promptname); need switch between views, should fragment