Modal dialog is blocking secondary thread run

My application spawns a secondary thread that performs long database tasks in background. For to access th开发者_开发百科e DB in the secondary thread I implemented this good solution, and it works perfectly until I open a modal dialog: the execution of secondary thread in this case it is blocked until I close the Dialog.

How do I avoid this? I want to leave the secondary thread free to perform database tasks for the antire life of my application, without interruption.