[API] Can't download or overwrite a file after uploading it 1 time(android) (have to reload the app)

package com.example.admin.prankclient;

import android.os.AsyncTask;
import android.util.Log;

import com.dropbox.core.DbxException;
import com.dropbox.core.DbxRequestConfig;
import com.dropbox.core.v2.DbxClientV2;
import com.dropbox.core.v2.files.FileMetadata;
import com.dropbox.core.v2.files.UploadErrorException;
import com.dropbox.core.v2.files.WriteMode;

import java.io.ByteArrayInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.Charset;

import static android.support.constraint.Constraints.TAG;

public class uploadFile {
    private static final String ACCESS_TOKEN = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
    private DbxRequestConfig config = null;
    DbxClientV2 client = null;
    private String command;
    private String foldername;
    uploadFile(String command, String foldername)
    {
        // Create Dropbox client
        config = new DbxRequestConfig("PrankClient");
        client = new DbxClientV2(config, ACCESS_TOKEN);
        this.command=command;
        this.foldername=foldername;
        Log.d(TAG,"------------------FIRST PHASE");
        new Uploading().execute();
        //Uploading run = new Uploading();
        //run.execute();

    }

    private class Uploading extends AsyncTask {


        @Override
        protected void onPostExecute(Object o) {
            Log.d(TAG,"------------------END");
            super.onPostExecute(o);
        }

        @Override
        protected Object doInBackground(Object[] objects) {
            Log.d(TAG,"------------------background");
            try {
                //String Dir = Environment.getExternalStorageDirectory().getAbsolutePath() + "/PrankClient";
                //InputStream in = new FileInputStream(Dir + "/Check.txt");
                InputStream in = new ByteArrayInputStream(command.getBytes(Charset.forName("UTF-8"))) ;

                FileMetadata metadata = client.files().uploadBuilder(foldername).withMode(WriteMode.OVERWRITE).uploadAndFinish(in);

                in.close();
            }
            catch (FileNotFoundException fne)
            {
                fne.printStackTrace();
            }
            catch (IOException ioe) {
                ioe.printStackTrace();
            } catch (UploadErrorException e) {
                e.printStackTrace();
            } catch (DbxException e) {
                e.printStackTrace();
            }

            return null;
        }
    }
}