I made the app but it keeps crashing whenever I press the button.

package com.pratikbaid3.additionapp;

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

public class MainActivity extends AppCompatActivity
{
    private TextView number;
    private Button additionButton;
    public int num;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        number=findViewById(R.id.number);
        additionButton=findViewById(R.id.additionButton);
        num=0;
        View.OnClickListener listener=new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                num+=1;
                number.setText(num);
            }
        };
        additionButton.setOnClickListener(listener);

    }
}