android - NPE error using Retrofit -

i want login in service called vid.me,https://api.vid.me/oauth/authorize post.but when try data log have nullpointerexception.i tryed make toast , have error too.i'm trying response code see did right or no.

my api class:

public interface videoapi {      @get("/videos/featured")     call<videos> getfeaturedvideo();      @get("/videos/new")     call<videos> getnewvideo();      @formurlencoded     @post("oauth/authorize")    call<signinresults>insertuser(@field("name") string name,                            @field("password") string password                            ); } 

my fragment:

public class feedfragment extends fragment {     edittext username;     edittext password;     button btnlogin;     public list<signinresult> signinresult;     public static final string root_url = "https://api.vid.me/";     public view oncreateview(layoutinflater inflater, viewgroup container,                              bundle savedinstancestate) {         view rootview = inflater.inflate(r.layout.fragment_feed, container, false); username = (edittext) rootview.findviewbyid(r.id.user_name_field);         password = (edittext) rootview.findviewbyid(r.id.password_field);         btnlogin = (button)rootview.findviewbyid(r.id.button_login);         btnlogin.setonclicklistener(new view.onclicklistener() {             @override             public void onclick(view v) {                 authorize();             }         });         return rootview;     }     public void authorize(){         retrofit retrofitadapter = new retrofit.builder()                 .addconverterfactory(gsonconverterfactory.create())                 .baseurl(root_url)                 .build();         final videoapi videoapi = retrofitadapter.create(videoapi.class);        call<signinresults> call = videoapi.insertuser(username.gettext().tostring(),password.gettext().tostring());         call.enqueue(new callback<signinresults>() {             @override             public void onresponse(call<signinresults> call, response<signinresults> response) {                 log.d("feedfragment", "status code = " + response.body().signinresults.get(0).getcode());             }              @override             public void onfailure(call<signinresults> call, throwable t) {              }         });      } } 

call<signinresponse> call = videoapi.insertuser(username_value, password_value); call.enqueue(new callback<signinresponse>() {       @override     public void onresponse(call<signinresponse> call, response<signinresponse> response) {         signinresponse results = response.body();         log.d("response ==>> ", new gsonbuilder().setprettyprinting().create().tojson(results));     }      @override     public void onfailure(call<signinresponse> call, throwable t) {      } });    @headers("content-type:application/x-www-form-urlencoded") @formurlencoded @post("/auth/create") call<signinresponse> insertuser(@field("email") string username,                             @field("password") string password );