Неустранимое исключение: Таймер-0?

Я создаю игру, которая рассчитывает время реакции. Я использую таймер, чтобы узнать время, необходимое пользователю для нажатия кнопки. По какой-то причине я получаю исключение Timer-0, когда пытаюсь запустить приложение.

LOGCAT

E/AndroidRuntime: FATAL EXCEPTION: Timer-0
Process: com.example.abz.layouts, PID: 16015
                  android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
                      at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6556)
                      at android.view.ViewRootImpl.invalidateChildInParent(ViewRootImpl.java:942)
                      at android.view.ViewGroup.invalidateChild(ViewGroup.java:5081)
                      at android.view.View.invalidateInternal(View.java:12713)
                      at android.view.View.invalidate(View.java:12649)
                      at android.view.View.invalidateDrawable(View.java:16788)
                      at android.widget.TextView.invalidateDrawable(TextView.java:5408)
                      at android.graphics.drawable.Drawable.invalidateSelf(Drawable.java:385)
                      at android.graphics.drawable.RippleDrawable.invalidateSelf(RippleDrawable.java:705)
                      at android.graphics.drawable.RippleDrawable.invalidateSelf(RippleDrawable.java:701)
                      at android.graphics.drawable.LayerDrawable.invalidateDrawable(LayerDrawable.java:896)
                      at android.graphics.drawable.DrawableWrapper.invalidateDrawable(DrawableWrapper.java:153)
                      at android.graphics.drawable.Drawable.invalidateSelf(Drawable.java:385)
                      at android.graphics.drawable.GradientDrawable.setColorFilter(GradientDrawable.java:837)
                      at android.graphics.drawable.DrawableWrapper.setColorFilter(DrawableWrapper.java:243)
                      at android.graphics.drawable.LayerDrawable.setColorFilter(LayerDrawable.java:1285)
                      at android.graphics.drawable.Drawable.clearColorFilter(Drawable.java:600)
                      at com.example.abz.layouts.HighLight.run(HighLight.java:33)
                      at java.util.Timer$TimerImpl.run(Timer.java:284)
D/EGL_emulation: eglMakeCurrent: 0xae424780: ver 3 0 (tinfo 0xae412ba0)
E/Surface: getSlotFromBufferLocked: unknown buffer: 0xaaac9820
Application terminated.

Вот мои классы Java

MainActivty.java

package com.example.abz.layouts;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.Random;
import java.util.Timer;
import java.util.TimerTask;



public class MainActivity extends AppCompatActivity {

    private long start_time;
    private long end_time;
    private Button finalButton;





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



    @Override
    protected void onStart() {
        super.onStart();



        // Create list of buttons
        ArrayList<Button> buttons = createButtonsArrayList();

        // Generates sequence of 5 random buttons
        RandomSequence randomSequence = new RandomSequence(buttons, 5, 9);

        // Add OnClickListener for last button
        finalButton = randomSequence.sequence.get(randomSequence.sequence.size() - 1);
        finalButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finalButtonClicked();
            }
        });

        // Start Game
        start_time = System.nanoTime();
        randomSequence.startSequence();

    }

    private void finalButtonClicked() {
        end_time = System.nanoTime();
        double diffInMillis = (end_time - start_time) / 1e6;
        finalButton.getBackground().clearColorFilter();
        Toast toast = Toast.makeText(this, "Reaction time: " + diffInMillis + " milliseconds.", Toast.LENGTH_LONG);
        toast.show();
    }

    private ArrayList<Button> createButtonsArrayList() {
        ArrayList<Button> buttons = new ArrayList<>();
        final Button button1 = (Button) findViewById(R.id.button1);
        final Button button2 = (Button)findViewById(R.id.button2);
        final Button button3 = (Button) findViewById(R.id.button3);
        final Button button4 = (Button) findViewById(R.id.button4);
        final Button button5 = (Button) findViewById(R.id.button5);
        final Button button6 = (Button) findViewById(R.id.button6);
        final Button button7 = (Button) findViewById(R.id.button7);
        final Button button8 = (Button) findViewById(R.id.button8);
        final Button button9 = (Button) findViewById(R.id.button9);


        buttons.add(button1);
        buttons.add(button2);
        buttons.add(button3);
        buttons.add(button4);
        buttons.add(button5);
        buttons.add(button6);
        buttons.add(button7);
        buttons.add(button8);
        buttons.add(button9);

        return buttons;
    }
}

HighLight.java

package com.example.abz.layouts;

import android.graphics.Color;
import android.graphics.PorterDuff;
import android.widget.Button;

import java.util.ArrayList;
import java.util.TimerTask;


public class HighLight extends TimerTask {

    private ArrayList<Button> sequence;
    private int sequencePosition;

   public HighLight(ArrayList<Button> sequence) {
        this.sequence = sequence;
        sequencePosition = 0;
    }

    public void run() {
        Button activeButton = sequence.get(sequencePosition);
        if (sequencePosition == 0) {
            activeButton.getBackground().setColorFilter(0xFF00FF00, PorterDuff.Mode.MULTIPLY);
        } else if (sequencePosition == sequence.size() - 1) {
            Button previousActiveButton = sequence.get(sequencePosition - 1);
            previousActiveButton.getBackground().clearColorFilter();
            activeButton.getBackground().setColorFilter(Color.RED, PorterDuff.Mode.MULTIPLY);
            // TODO: PLAY SOUND FOR FINAL BUTTON
            this.cancel();
        } else {
            Button previousActiveButton = sequence.get(sequencePosition - 1);
            previousActiveButton.getBackground().clearColorFilter();
            activeButton.getBackground().setColorFilter(0xFF00FF00, PorterDuff.Mode.MULTIPLY);
        }
        sequencePosition++;
    }
}

RandomSequence.java

package com.example.abz.layouts;

        import android.os.Looper;
        import android.widget.Button;

        import java.util.ArrayList;
        import java.util.Random;
        import java.util.Timer;
        import java.util.logging.Handler;


public class RandomSequence {

    public ArrayList<Button> sequence;
    private ArrayList<Button> buttons;
    private int length;

    public RandomSequence(ArrayList<Button> buttons, int low, int high) {
        this.buttons = buttons;

        length = newRandomNumber(low, high);
        sequence = generateRandomSequence();

    }

    public void startSequence() {
        Timer timer = new Timer();
        timer.schedule(new HighLight(sequence), 0, 1500);
    }

    private ArrayList<Button> generateRandomSequence() {
        ArrayList<Button> randomSequence = new ArrayList<>();
        for (int i = 1; i <= length; i++) {
            int random = newRandomNumber(0, buttons.size());
            Button button = buttons.get(random);
            // Ensure not same number in a row
            while (randomSequence.size() > 0 && button.getId() == randomSequence.get(randomSequence.size() - 1).getId()) {
                random = newRandomNumber(0, buttons.size());
                button = buttons.get(random);
            }
            randomSequence.add(buttons.get(random));
        }
        return randomSequence;
    }

    private int newRandomNumber(int low, int high) {
        Random r = new Random();
        return r.nextInt(high - low) + low;
    }
}

Я пытался найти решения, но безуспешно.