@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        final View vista = inflater.inflate(R.layout.fragment_service, container, false);

        btnback = (ImageButton) vista.findViewById(R.id.btnback);
        btnback.setVisibility(View.VISIBLE);
        btnback.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getActivity().finish();
            }
        });

return vista;
}

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageButton.setVisibility(int)' on a null object reference
                      at com.example.matia.tonum.Fragment.ServiceFragment.onCreateView(ServiceFragment.java:55)
                      at android.support.v4.app.Fragment.performCreateView(Fragment.java:2346)
                      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1428)
                      at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1759)
                      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1827)
                      at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:797)
                      at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2596)
                      at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2383)
                      at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)
                      at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2245)
                      at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:703)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:135)
                      at android.app.ActivityThread.main(ActivityThread.java:5258)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:372)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)