Multithreading issues... I see, the solution is mentioned in the exception itself. You can only modify the view in the thread you created it. Move this line out of the new thread
fmd.Data(ser.FindClients().interact.get(a).fileNames);
You need some concurrency utilities to solve this, such as Exchanger or Semaphore. Try to notify the main thread that a background operation has finished and data has been returned. Then set the data of the view in the main thread.
Here main thread means the thread that the thread that created the view.