File picker run time exception

Question

i'm creating a android filepicker project. the app works perfectly in the emulator but when i connected to my device it won't run. i faced a nullpointer exception for my device, but when i run that project in emulator it was perfectly executed. i don't know what the exact problem, can anyone tell me what's this?

exception look like this:

java.lang.NullPointerException: Attempt to get length of null array
        at ir.sohreco.androidfilechooser.FileChooser.loadItems(FileChooser.java:507)
        at ir.sohreco.androidfilechooser.FileChooser.onCreateView(FileChooser.java:446)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2600)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:881)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2079)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1869)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2663)
        at androidx.fragment.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManagerImpl.java:2613)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:246)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:542)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:201)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1433)
        at android.app.Activity.performStart(Activity.java:7978)

filepacker.java

public class filepicker extends AppCompatActivity {
    private final static int READ_EXTERNAL_STORAGE_PERMISSION_REQUEST_CODE = 13;

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

        int permissionCheck = ContextCompat.checkSelfPermission(this,
                Manifest.permission.READ_EXTERNAL_STORAGE);
        if (permissionCheck != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                    READ_EXTERNAL_STORAGE_PERMISSION_REQUEST_CODE);
        } else {
            addFileChooserFragment();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == READ_EXTERNAL_STORAGE_PERMISSION_REQUEST_CODE) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                addFileChooserFragment();
            }
        }
    }

    private void addFileChooserFragment() {
        FileChooser.Builder builder = new FileChooser.Builder(FileChooser.ChooserType.FILE_CHOOSER,
                new FileChooser.ChooserListener() {
                    @Override
                    public void onSelect(String path) {
                        Toast.makeText(filepicker.this, path, Toast.LENGTH_SHORT).show();
                    }
                });
        try {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.file_chooser_fragment_container_framelayout, builder.build())
                    .commit();
        } catch (ExternalStorageNotAvailableException e) {
            Toast.makeText(this, "There is no external storage available on this device.",
                    Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        }
    }
}