DropNet exception

So, I'm new to using an external API for projects, but I got an idea and would like to know if I can upload files to Dropbox correctly. I went through the steps to get the key and token, and I started testing, but I ran into an error:

An unhandled exception of type "System.ArgumentNullException" occurred in DropNet.dll Additional information: The value cannot be null.

Update: In the future, in case of an error, the actual parameter equal to null will be "userLogin" **

This is my code before the error: // using Dropnet;

DropNetClient _client = new DropNetClient("API_KEY", "API_SECRET", DropNetClient.AuthenticationMethod.OAuth1);

Error code:

var url = _client.BuildAuthorizeUrl();

And my code following the error:

Process.Start(url);
_client.GetAccessTokenAsync((accessToken) =>
    {
        _client = new DropNetClient("API_KEY", "API_SECRET", accessToken.Token, accessToken.Secret);
    },
    (error) =>
    {
        MessageBox.Show(error.Message);
    });
try
{
    _client.UploadFile("/", "test.txt", ReadFile(@"D:\Classes\Documents\test.txt"));

    MessageBox.Show("Successfully uploaded to Dropbox.", "Uploaded to Dropbox");
}
catch (Exception dropboxEx)
{
    MessageBox.Show("Error: " + dropboxEx.Message);
}

, - , , ? OAuth uri , , , , , Dropbox:

, , , !