java.lang.IllegalArgumentException: String 'path' does not match pattern

I just download the dropbox API and integrate into my android code, but it shows error that the path is not match and I don't know where to fix it.

FATAL EXCEPTION: AsyncTask #2
 Process: com.example.inspiron.smartdoorapp2, PID: 16439
 java.lang.RuntimeException: An error occurred while executing doInBackground()
     at android.os.AsyncTask$3.done(AsyncTask.java:309)
     at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
     at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
     at java.util.concurrent.FutureTask.run(FutureTask.java:242)
     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
     at java.lang.Thread.run(Thread.java:818)
  Caused by: java.lang.IllegalArgumentException: String 'path' does not match pattern
     at com.dropbox.core.v2.files.ListFolderArg.<init>(ListFolderArg.java:68)
     at com.dropbox.core.v2.files.ListFolderArg.<init>(ListFolderArg.java:100)
     at com.dropbox.core.v2.files.DbxUserFilesRequests.listFolder(DbxUserFilesRequests.java:1384)
     at com.example.inspiron.smartdoorapp2.ListFolderTask.doInBackground(ListFolderTask.java:43)
     at com.example.inspiron.smartdoorapp2.ListFolderTask.doInBackground(ListFolderTask.java:12)
     at android.os.AsyncTask$2.call(AsyncTask.java:295)
     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
     at java.lang.Thread.run(Thread.java:818)