AlertDialog未关闭颤动

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private AlertDialog alertDialog;

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

        Button showDialogButton = findViewById(R.id.show_dialog_button);
        showDialogButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showDialog();
            }
        });
    }

    private void showDialog() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("提示");
        builder.setMessage("这是一个AlertDialog");
        builder.setPositiveButton("确定", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                // 关闭对话框
                dialog.dismiss();
            }
        });
        builder.setNegativeButton("取消", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                // 关闭对话框
                dialog.dismiss();
            }
        });

        alertDialog = builder.create();
        alertDialog.show();
    }
}