Tambahkan pada bagian oncreate

MainActivity.java

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FacebookSdk.sdkInitialize(getApplicationContext());
        callbackManager=CallbackManager.Factory.create();
        setContentView(R.layout.activity_main);
        tvInfo=(TextView)findViewById(R.id.tvInfo);
        btnLogin=(LoginButton)findViewById(R.id.btnLogin);