Rects and regions that you create in rect palette are global variables.
JASS:
globals
trigger gg_trg_Melee_Initialization = null
rect gg_rct_Rect_000 = null
endglobals
Global variables can't be created while game process.
But I think that you can create a local rect in the trigger where town hall finished construction wtih this native function:
(native Rect takes real minx, real miny, real maxx, real maxy returns rect)
JASS:
local rect r=Rect(0.,0.,0.,0.)