Attempt to read from field 'java.util.List com.dropbox.client2.DropboxAPI$Entry.contents' on a null

I have an android application which has been working perfectly until now. Recently, I'm having an error regarding the dropbox download. I've been using this app and the dropbox download for months but I never had this error yet. I tried to solve it but I found nothing similar. Is something wrong with dropbox or I made some mistake?

My question is why dirent = dropboxAPI.metadata("/", 1000, null, true, null);

dirent becomes null?

Thanks very much in advance!

Code

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();
}
}

Error: 

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)