When ever I try to open the app, it doesn't work...

package harrycourt.funfactsapp;

import android.content.DialogInterface;
import android.graphics.Color;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RelativeLayout;
import android.widget.TextView;

import java.util.Random;

public class FunFactsActivity extends AppCompatActivity {
    private FactBook mFactBook = new FactBook();
    // Declare our view vars
    private TextView mFactTextView;
    private Button mShowFactButton;
    private RelativeLayout mRelativeLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fun_facts);

        // Assign views from the layout file to the corresponding vars
        mFactTextView = (TextView) findViewById(R.id.factTextView);
        mShowFactButton = (Button) findViewById(R.id.showFactButton);
        mRelativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout);

        View.OnClickListener listener = new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String fact = mFactBook.getFact();

                // Update screen with fact.
                mFactTextView.setText(fact);
                mRelativeLayout.setBackgroundColor(Color.RED);
            }
        };
        mShowFactButton.setOnClickListener(listener);
    }
}

package harrycourt.funfactsapp;

import java.util.Random;

/**
 * Created by Harry on 1/05/2017.
 */

public class FactBook {
    // Fields (Member vars)  Propities about the object.
    String[] mFacts = {
            "Ants stretch when they wake up in the morning.",
            "Ostriches can run faster than horses.",
            "Olympic gold medals are actually made mostly of silver.",
            "You are born with 300 bones; by the time you are an adult you will have 206.",
            "It takes about 8 minutes for light from the Sun to reach Earth.",
            "Some bamboo plants can grow almost a meter in just one day.",
            "The state of Florida is bigger than England.",
            "Some penguins can leap 2-3 meters out of the water.",
            "On average, it takes 66 days to form a new habit.",
            "Mammoths still walked the earth when the Great Pyramid was being built.",
            "This app was made possible with the help of Treehouse!" };

    // Methods - Actions the object can take
    public String getFact() {


        String fact = "";
        // Randomly select a fact.
        Random randomGenerator = new Random();
        int randomNumber = randomGenerator.nextInt(mFacts.length);
        fact = mFacts[randomNumber];

        return  fact;
    }
}

<?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"
    android:id="@+id/relativeLayout"
    android:background="#51b46d"
    tools:context="harrycourt.funfactsapp.FunFactsActivity"
    tools:layout_editor_absoluteX="0dp"
    tools:layout_editor_absoluteY="81dp">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="40dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="40dp"
        android:text="Did you know..."
        android:textColor="@color/TransparentWhite"
        android:textSize="24sp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:layout_constraintLeft_creator="1"
        tools:layout_constraintTop_creator="1"/>

    <TextView
        android:id="@+id/factTextView"
        android:layout_width="345dp"
        android:layout_height="57dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:text="This app was made by the help of Treehouse!"
        android:textColor="@android:color/white"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.521"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:layout_constraintBottom_creator="1"
        tools:layout_constraintLeft_creator="1"
        tools:layout_constraintRight_creator="1"
        tools:layout_constraintTop_creator="1"/>

    <Button
        android:id="@+id/showFactButton"
        android:layout_width="0dp"
        android:layout_height="55dp"
        android:layout_marginBottom="55dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:background="@android:color/white"
        android:text="Next Fact"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        tools:layout_constraintBottom_creator="1"
        tools:layout_constraintLeft_creator="1"
        tools:layout_constraintRight_creator="1"/>

</android.support.constraint.ConstraintLayout>