我通过检查用户是否已授予外部存储权限，如果未授予权限，则通过使用请求权限来解决问题:

I solved the problem by checking if the user has granted permission of external storage and request permision if not granted by using:

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                if (this.checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
                     this.requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, WRITE_REQUEST_CODE);
            }

所有这些都是 android 特定的代码，因此驻留在 AndroidLauncher 类中.

All of this is android-specific code and thus resides in the AndroidLauncher class.