qt window closed but flowgraph not killed

I'm new with GNU Radio. Usualy when I close a Qt window, the flowgraph
stops, but when I use a USRP source or sink, the window closes but the
flowgraph still run and I need to close it with the kill button.
I have googled a lot but I can't find an explanation.
Can someone give me some ideas
Thanks a lot
Juanjo