The application does not work on redmi 3s, what is the reason?

Question

Android

Good day. Recently, I ran into a problem. From one of the users who bought my application on google play, I received a message that the application does not work on redmi 3s.
The problem is that I have absolutely no idea what could be the reason. As the user reported, the error occurs when clicking on the button, which leads to the activity with the following code:

package com.magical3000.sweet_dreams;

import android.os.Bundle;
import android.content.Intent;
import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.webkit.WebView;
import android.widget.Button;

public class activity1 extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_1);
        Dreaming();
    }

    MediaPlayer mPlayer;
    int i=0, limit=90, nowsound=R.raw.relax, check=0;

    public void onClicka1(View view) {
        mPlayer.stop();
        Intent intent = new Intent(activity1.this, ChooseActivity.class);
        startActivity(intent);
    }

    private void Dreaming() {
        mPlayer=MediaPlayer.create(this, nowsound);
        mPlayer.start();
        mPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
            @Override
            public void onCompletion(MediaPlayer mp) {
                Dreaming1();
            }
        });
    }

    private void Dreaming1() {
        if (i<limit)
        {
            i=i+1;
            mPlayer.start();
        }
        else
        {
            if (check==0 || check==2 || check==4)
            {
                Dreaming2();
            }
            else {
                if (check == 1 || check == 3) {
                    Dreaming3();
                }
                else {
                    if (check == 5) {
                        i = 0;
                        limit = 90;
                        nowsound = R.raw.relax;
                        mPlayer.start();
                    }
                }
            }
        }
    }

    private void Dreaming2() {
        check=check+1;
        nowsound=R.raw.sleep1;
        i=0;
        limit=270;
        mPlayer.start();
    }

    private void Dreaming3() {
        check=check+1;
        nowsound=R.raw.sleep2;
        i=0;
        limit=270;
        mPlayer.start();
    }

}

Please help me find the cause of the error and fix it.