Windows form obliterates Dialog System window after closing form

I'm calling managed code from native.  The managed code displays a "Please wait" dialog using the ShowDialog method and then does some processing.   When the processing is complete, I do the Close method on the form and return to my native code.  I'm starting a thread to do the processing then showing the "Please wait" form and am using a Timer component to determine when processing is done so I can close it and exit back to my native code which uses Dialog System for the UI.

After closing the form,  the text on the underlying native Dialog window is obliterated.  The text returns if I refresh the window but I can't do this because I'm calling the managed code from some other program.  This is leaving a blank rectangle on my Dialog System screen, which is the same size as the Windows form.

Is there a way to prevent this from happening ?