一调用CountDownTimer.Cancel()方法，我的计时器应用程序就崩溃了，它显示的对象引用为空，尽管我做得正确，但我找不到问题。请，某人，看看下面的代码：（我也附上logcat）

原因：java.lang.NullPointerException：尝试对空对象引用调用虚拟方法“void android.os.CountDownTimer.Cancel（）”

package com.example.rajdeepkgautam.itimer;

import android.media.MediaPlayer;
import android.os.CountDownTimer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.SeekBar;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

int max = 600;
int start = 30;

TextView timerTextView;
SeekBar timerSeekBar;
boolean timerActive = false;
Button runButton;
CountDownTimer countDownTimer;

public void resetTimer() {

    timerTextView.setText("0:30");
    timerSeekBar.setProgress(start);
    timerSeekBar.setEnabled(true);
    countDownTimer.cancel();
    runButton.setText("RUN!");
    timerActive = false;

}

public void buttonClicked(View view) {

    if(timerActive) {

        resetTimer();

    } else {

            timerActive = true;
            runButton.setText("STOP!");
            timerSeekBar.setEnabled(false);

            CountDownTimer countDownTimer = new CountDownTimer((timerSeekBar.getProgress()*1000) + 100, 1000) {
                @Override
                public void onTick(long l) {

                    TimerUpdate((int) l/1000);

                }

                @Override
                public void onFinish() {

                    MediaPlayer mediaPlayer = MediaPlayer.create(getApplicationContext(), R.raw.horn);
                    mediaPlayer.start();
                    resetTimer();

                }
            }.start();



        }
}



public void TimerUpdate(int TimeLeft) {

    int minutes = TimeLeft/60;
    int seconds = TimeLeft - (minutes*60);

    if(seconds <=9) {

        timerTextView.setText(Integer.toString(minutes) + ":" + "0" + Integer.toString(seconds));

    } else {

        timerTextView.setText(Integer.toString(minutes) + ":" + Integer.toString(seconds));

    }


}

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

    timerSeekBar = findViewById(R.id.timerSeekBar);
    timerTextView = findViewById(R.id.timerTextView);
    runButton = findViewById(R.id.runButton);



    timerSeekBar.setMax(max);
    timerSeekBar.setProgress(start);


    timerSeekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
        @Override
        public void onProgressChanged(SeekBar seekBar, int i, boolean b) {

            TimerUpdate(i);

        }

        @Override
        public void onStartTrackingTouch(SeekBar seekBar) {

        }

        @Override
        public void onStopTrackingTouch(SeekBar seekBar) {

        }
    });

}
}