According to: this There's no ActivityInstance variable predefined, so I assume you have created it yourself and it might be the case that it's not correct. Try using CurrentActivityInstance instead.

According to: this There's no ActivityInstance variable predefined, so I assume you have created it yourself and it might be the case that it's not correct. Try using CurrentActivityInstance instead.

Try this:

(ComponentData)CoreServiceClient.Read(CurrentActivity.WorkItems[0].Subject.IdRef‌​, new ReadOptions());