Seems like, at the time, Drago forgot Arrays are 0-indexed. How this went unnoticed for years is beyond me.
For the sake of keeping the fix easy, find this around line 38 and make sure it looks like this
def mpit() @mpit ||= [nil, 0, 0, 1, 1, 0, 0] end
@Drago: Problem lines are
# Scene_Map#main
@mpit.visible, @mpit.contents_opacity = true, $game_system.mpit[6]
# Scene_Map#update
@mpit.contents_opacity, @mpit.visible= $game_system.mpit[6], true
$game_system.mpit[6] -= 3
The
mpit variable was only an array of 6 elements, hence mpit[6] returned nil.