android - App crashes when calling setValue -

in main activity, work working , there no problem firebase. in other activities, firebase sends data setvalue application crashes after that.why?

public class main_activity extends appcompatactivity {       static string firstoption;     static string optionfour;     static string secondoption;     static string thirdoption;     static string thequestion;     static string correctanswer;     public static string[] str = new string[100];     private  int index_yoni=0;     private textview mainpagetitle;     private firebaseauth firebaseauth;     private firebase fire;     private string uid;      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main_activity);            firebase.setandroidcontext(main_activity.this);         fire = new firebase("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/");            mainpagetitle=(textview) findviewbyid(r.id.textview3);              fire.addvalueeventlistener(new valueeventlistener() {             public void ondatachange(datasnapshot datasnapshot) {                 firebaseauth=firebaseauth.getinstance();                 firebaseuser user=firebaseauth.getcurrentuser();                if (firebaseauth.getcurrentuser() != null) {                mainpagetitle.settext(datasnapshot.child("userinformation").child(user.getuid()).child("name").getvalue()+" "+"'welcome");            }           else            {                mainpagetitle.settext("'welcom game");            }              }     public void oncancelled(firebaseerror firebaseerror) {         log.e("the error ....", firebaseerror.getmessage());     }            });           fire.addvalueeventlistener(new valueeventlistener() {             public void ondatachange(datasnapshot datasnapshot) {                 (datasnapshot sportsnapshot : datasnapshot.child(typesoftests.topic).getchildren()) {                     sportsquestions sportsquestions = sportsnapshot.getvalue(sportsquestions.class);                     firstoption = sportsquestions.getfirstoption();                     optionfour = sportsquestions.getoptionfour();                     secondoption = sportsquestions.getsecondoption();                     thirdoption = sportsquestions.getthirdoption();                     correctanswer = sportsquestions.getcorrectanswer();                     thequestion = sportsquestions.getthequestion();                     increment();                 }             }             public void oncancelled(firebaseerror firebaseerror) {                 log.e("the error ....", firebaseerror.getmessage());             }         }         @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_profile2);          firebase.setandroidcontext(profileactivity.this);         fire= new firebase("https://triviaproject-4c91e.firebaseio.com/");          fire.child("userinformation").child(user.getuid()).setvalue(userinformation); 

fire.addvalueeventlistener(new valueeventlistener() {         public void ondatachange(datasnapshot datasnapshot) {             (datasnapshot sportsnapshot : datasnapshot.child(typesoftests.topic).getchildren()) {                 sportsquestions sportsquestions = sportsnapshot.getvalue(sportsquestions.class);                 firstoption = sportsquestions.getfirstoption();                 optionfour = sportsquestions.getoptionfour();                 secondoption = sportsquestions.getsecondoption();                 thirdoption = sportsquestions.getthirdoption();                 correctanswer = sportsquestions.getcorrectanswer();                 thequestion = sportsquestions.getthequestion();                 increment();             }           //you need add add stop           fire.removeeventlistener(this)         }