空对象引用上的'java.util.List com.dropbox.client2.DropboxAPI $ Entry.contents'

Question

我有一个Android应用程序，一直运作到现在为止。最近，我有关于Dropbox下载的错误。我已经使用这个应用程序和Dropbox下载几个月但我从来没有这个错误。我试图解决它，但我发现没有类似的东西。 Dropbox出了什么问题，或者我犯了一些错误？

修改的

我知道空对象引用是什么，我的问题是为什么

dirent = dropboxAPI.metadata（“/”，1000，null，true，null）;

dirent变为空？

非常感谢！

public void downAndLoad() {
AsyncTask<String, Void, Void> asyncTask = new AsyncTask<String, Void, Void>() {
        @Override@Override
        protected Void doInBackground(String... strings) {

            AndroidAuthSession session = null;
            try {
                session = buildSession();
            } catch (DropboxException e) {
                e.printStackTrace();
            }
            dropboxAPI = new DropboxAPI<>(session);
            String[] fnames = null;
            DropboxAPI.Entry dirent = null;
            try {
                dirent = dropboxAPI.metadata("/", 1000, null, true, null);
            } catch (DropboxException e) {
                e.printStackTrace();
            }
        }

            ArrayList<DropboxAPI.Entry> files = new ArrayList<DropboxAPI.Entry>();
            ArrayList<String> dir = new ArrayList<String>();


            int i = 0;
            for (DropboxAPI.Entry ent : dirent.contents) {
                files.add(ent);// Add it to the list of thumbs we can choose from
                //dir = new ArrayList<String>();
                dir.add(files.get(i++).path);
            }

            fnames = dir.toArray(new String[dir.size()]);           

错误：

10-01 00:31:08.963 11769-11961/com.example.brobert.tonda E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #4
                                                                        Process: com.example.brobert.tonda, PID: 11769
                                                                        java.lang.RuntimeException: An error occurred while executing doInBackground()
                                                                            at android.os.AsyncTask$3.done(AsyncTask.java:318)
                                                                            at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
                                                                            at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
                                                                            at java.util.concurrent.FutureTask.run(FutureTask.java:242)
                                                                            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:243)
                                                                            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
                                                                            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
                                                                            at java.lang.Thread.run(Thread.java:762)
                                                                         Caused by: java.lang.NullPointerException: Attempt to read from field 'java.util.List com.dropbox.client2.DropboxAPI$Entry.contents' on a null object reference
                                                                            at com.example.brobert.tonda.dldb_lp$1.doInBackground(dldb_lp.java:149)
                                                                            at com.example.brobert.tonda.dldb_lp$1.doInBackground(dldb_lp.java:126)
                                                                            at android.os.AsyncTask$2.call(AsyncTask.java:304)
                                                                            at java.util.concurrent.FutureTask.run(FutureTask.java:237)
                                                                            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:243) 
                                                                            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133) 
                                                                            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607) 
                                                                            at java.lang.Thread.run(Thread.java:762)