How do I implement a login screen using the Twitter Kit for Android?

I'm trying to implement a Twitter feature in my custom Android app using their new Twitter Kit 3 API, but I can't seem to get it to work.

I just followed this example: https://dev.twitter.com/twitterkit/android/log-in-with-twitter

My MainActivity class is as follows:

package gib.twitterpruebacliente;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

import com.twitter.sdk.android.core.Callback;
import com.twitter.sdk.android.core.Result;
import com.twitter.sdk.android.core.Twitter;
import com.twitter.sdk.android.core.TwitterException;
import com.twitter.sdk.android.core.TwitterSession;
import com.twitter.sdk.android.core.identity.TwitterLoginButton;

public class MainActivity extends AppCompatActivity {

private TwitterLoginButton loginButton;

@Override
protected void onCreate(Bundle savedInstanceState) {
    Twitter.initialize(this);

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    loginButton = (TwitterLoginButton) findViewById(R.id.login_button);
    loginButton.setCallback(new Callback<TwitterSession>() {
        @Override
        public void success(Result<TwitterSession> result) {
            // Do something with result, which provides a TwitterSession for making API calls
            Toast.makeText(MainActivity.this, "IT WORKS!", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void failure(TwitterException exception) {
            // Do something on failure
            Toast.makeText(MainActivity.this, "FAIL!", Toast.LENGTH_SHORT).show();
        }
    });
    }


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

    // Pass the activity result to the login button.
    loginButton.onActivityResult(requestCode, resultCode, data);
}
}

      

        
        
        
      

    

My main XML activity only has tweeterLogLogButton:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="gib.twitterpruebacliente.MainActivity">


    <com.twitter.sdk.android.core.identity.TwitterLoginButton
        android:id="@+id/login_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</android.support.constraint.ConstraintLayout>

      

        
        
        
      

    

I have already added permission to the manifest:

<uses-permission android:name="android.permission.INTERNET" />

      

        
        
        
      

    

And I added KEYS for my application to the strings folder:

   <string name="com.twitter.sdk.android.CONSUMER_KEY">suP4TlwNI7xpwh3sK4x5fKldL</string>
<string name="com.twitter.sdk.android.CONSUMER_SECRET">oaAzoqtMlu5ZtY40LPdzfZTk5wNSTg6Lrnkgx5MPc4TTLbfzMZ</string>

      

        
        
        
      

    

But the app always goes into the crash section. How can I do?