Android Facebook - 获取空个人资料

第一次使用 Facebook SDK。我无法获取用户个人资料。它始终为空。怎么了？

btnFbWidget = (LoginButton) findViewById(R.id.btnFbWidget);
btnFbWidget.setReadPermissions("public_profile");
// Callback registration
btnFbWidget.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
    @Override
    public void onSuccess(LoginResult loginResult) {
        Profile profile = Profile.getCurrentProfile();
        if (profile != null)
            Toast.makeText(getApplicationContext(), "LogIN as " + profile.getName(), Toast.LENGTH_SHORT).show();
        else 
            Toast.makeText(getApplicationContext(), "nulll", Toast.LENGTH_SHORT).show();
    }
                /*...*/
});

也用这个方法，......

 @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        callbackManager.onActivityResult(requestCode, resultCode, data);
    }