Android permission granting keep failing with massage "Package installer has stopped"

Question

I want to use ZXingScannerView, to read QR-code with my app. I need to grant camera permission to use ZXingScannerView, right? The view doesn't work for some reason which I don't know.

Anyway, I thought so, so I tried to grant permission, but it keeps failing, saying "Package installer has stopped". I think the bug is occur on line if(permissioncheck == PackageManager.PERMISSION_GRANTED)

Here is entire code

ZXingScannerView mScanview;

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

    int permissioncheck = ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA);
    if(permissioncheck == PackageManager.PERMISSION_GRANTED){
        Toast.makeText(this, "ok", Toast.LENGTH_SHORT).show();
    }


    else {
        Toast.makeText(this, "not ok", Toast.LENGTH_SHORT).show();
        if(ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.CAMERA)) {
            Toast.makeText(this, "ok?", Toast.LENGTH_SHORT).show();
        }
        else {
            ActivityCompat.requestPermissions(this, new String[] {Manifest.permission.CAMERA}, 1);
        }
    }




    Button button = (Button)findViewById(R.id.qr_button);
    button.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View view){

            Toast.makeText(getApplicationContext(), "OK", Toast.LENGTH_SHORT).show();


            mScanview = new ZXingScannerView(getApplicationContext());
            setContentView(mScanview);

        }
    });

}

@Override
public void onRequestPermissionsResult(int reqCode, String permissions[], int[] grantResult){
    switch (reqCode){
        case 1: {
            if(grantResult.length > 0 && grantResult[0] == PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(this, "ok", Toast.LENGTH_SHORT).show();
            }
            else {
                Toast.makeText(this, "not ok", Toast.LENGTH_SHORT).show();
            }

            return;
        }
    }
}

@Override
public void handleResult(Result result){
    Log.v("TAG", result.getText());
    Log.v("TAG", result.getBarcodeFormat().toString());
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle("Scan Result");
    builder.setMessage(result.getText());
    AlertDialog dialog = builder.create();
    dialog.show();
}

@Override
public void onPause() {
    super.onPause();
    mScanview.stopCamera();
}

Or, if you know better way of implementing QR-code reader in app, Could you let me know about that?