Problem with SpriteGrabber and Storyboard

Hi,
I have 2 scenes, the first requires SpriteGrabber. In this one when I leave the scene I destroy the sheet:

local grabber=require("SpriteGrabber")  
local sheet=grabber.grabSheet("sprites",".jpg")  
  
function scene:destroyScene(evt)  
 local group = self.view  
  
 sheet:remove()  
 sheet=nil  
end  

The 1st scene works and leave without bug and when I arrives to the 2nd scene, which use nothing special, all is ok but when I come back to the 1st scene, I get the error "attempt to index the upvalue ‘sheet’

Why it’s not possible to use twice the same scene when removing the sheet? By the way if I juste remove the sheet without niling it, the simulator crash…

Tested with Build 2011.704 [import]uid: 107239 topic_id: 26840 reply_id: 326840[/import]