通过 Android Library 从 Owncloud 下载图片失败

如何解决通过 Android Library 从 Owncloud 下载图片失败

我想使用来自 github (https://github.com/owncloud/android-library/blob/master/owncloudComLibrary/src/main/java/com/owncloud/android/lib/resources/files/DownloadRemoteFileOperation.java) 的预定义代码和我自己的代码将一些文件从自己的 OwnCloud-Server 下载到我的 android 设备`

Downloader downloader = new Downloader("joy-3147070.jpg","/sdcard/DCIM");

    OwnCloudClient client = new OwnCloudClient(Uri.parse("URL")); -> the URL has the following format: "https://XX.com/XX"
    client.setCredentials(OwnCloudCredentialsFactory.newBasicCredentials("username","password"));
    download = findViewById(R.id.button_download);
    download.setonClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            downloader.run(client);`

My code using the downloader from github as a picture

但是代码的执行被下面的异常“java.lang.NullPointerException: HttpClient.getokHttpClient() must not be null”回复。 代码片段中可能存在什么问题，我该如何解决？

提前致谢！