From Xojo Documentation
The Run method has been called. The threaded code is placed in this event handler. Methods called from here also run in the thread.
MacOS apps cannot access the user interface from within a thread. Doing so raises a ThreadAccessingUIException. Instead, use the Thread.AddUserInterfaceUpdate method in conjunction with the Thread.UserInterfaceUpdate event. See the Thread.UserInterfaceUpdate event for an example.