In the Spriteset_Map#initialize method in the script, you need to move
init_options up more so that it looks like this
#--------------------------------------------------------------------------
# * Initialize Object
# Rewritten to call a map with mode7
#--------------------------------------------------------------------------
alias initialize_mode7_spriteset_map initialize
def initialize
init_options
if !$game_system.mode7
initialize_mode7_spriteset_map
return
end
#... rest of the code
Reason: $game_system.mode7 = false only when init_options is called. When going from a Mode-7 map to a normal one, $game_system.mode7 was still true, and would skip the original aliased method, building the tilemap with the Mode-7 rewrite and, on the first frame of updating the spriteset, error out.