stringwidth() pb / center label text

You get the error because you don't have a Buffer to draw to. Inserting the following lines after your 'SetFont' statement should fix it:

	canvas = CreateCanvas ( 0,0,10,10,Desktop() )
	HideGadget canvas
	SetBuffer CanvasBuffer( canvas )

This is probably a bug tho' (at least as far as I can tell), so you might want to post it in the bugreport forum aswell.