AsyncTask返回null值

Question

我在获取Dropbox shareLink时遇到问题。我试过在这里根据一些答案改变了代码，但输出仍然相同。我不知道这些代码的哪一部分是错误的

这是我的代码：

public class loadMetadata extends AsyncTask<String, Void, String> {

    private DropboxAPI<?> mApi;
    private String mPath;
    //private UploadRequest mRequest;
    private Context mContext;
    private String[] fnames;
    String shareAddress = null;
    //private int i = 0;

    public loadMetadata(Context context, DropboxAPI<?> api, String dropboxPath) {
        // We set the context this way so we don't accidentally leak activities
        mContext = context.getApplicationContext();

        //mFileLen = file.length();
        mApi = api;
        mPath = dropboxPath;
        //mFile = file;
    }

        protected String doInBackground(String... String) {
            String shareAddress = null;
            int i = 0;
              try {
                    Entry dropboxDir = mApi.metadata(mPath, 1000, null, true, null);


                    for (Entry ent : dropboxDir.contents)
                    {
                        if (!ent.isDir) {
                            DropboxLink shareLink = mApi.share(ent.path);
                            //shareAddress = shareLink.url.replaceFirst("https://www", "https://dl");
                            shareAddress = shareLink.url;
                            Log.d("Dropbox","dropbox share link " + shareAddress);
                            //fnames[i] = shareAddress;
                        } 
                        i++;
                    }



                } catch (DropboxException e) {

                }

            return shareAddress;


    }  


        protected void onPostExecute(String result) 
        {
            if(result != null)
            {
            Toast.makeText(mContext, result, Toast.LENGTH_LONG).show();

            }

        }        
}