The problem isn't the AI, but rather autosave. While saving the game also saves a scenario file and the crash is in that file. It's perfectly harmless unless you plan to use the file, though it can be annoying and naturally it should be fixed.
PHP:
f.write("\t\tUnitAIType=%s\n" %(gc.getUnitAIInfo(unit.getUnitAIType()).getType()))
Looks like the offending line is this one. Apparently gc.getUnitAIInfo returns None rather than what it was supposed to return. Without knowing anything else, this could be difficult to fix.
Would you please upload Documents\My Games\Civ4Colonization\Saves\WorldBuilder\WBQuickSave.ColonizationWBSave
Luckily it seems that critical information is saved just before the crash and that file will tell us information about the unit, which causes the crash.
A savegame could also be useful.