if i have seen it correct you are true cause there is no check for canceling the uprgrade wall funktion if your gold or elix goes unter the minimum.
there should be a resource check after upgrading a wall and the loop shoud restart
alternatively you can to it this way:
wall cost is for lvl 7 walls 200.000
set minimum to upgrade walls to 250k-399k so there will always be one wall upgraded and still 50-199k left for searching a new town to attack.
this will be messed up if you have a wall of 2 pieces and 401k gold but is working better as with a high amount.