TypeError: Error #1009?

[braskez]

Alright so I'm working on making a skill calculator for this a game. This one I am working on right now has multiple skill calculators in a single flash file, but for some reason that I can't figure out, the skill calculator won't actually calculate. Here is a pastebin length to the AS code

http://pastebin.com/xB6xzJFd

Alright, I apoligize for its length, but I really have no idea what I'm doing wrong with this one. Here is the output:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at allInOneCalc_fla::MainTimeline/changeCalc()
	at flash.events::EventDispatcher/dispatchEventFunction()
	at flash.events::EventDispatcher/dispatchEvent()
	at fl.controls::ComboBox/onListChange()
	at flash.events::EventDispatcher/dispatchEventFunction()
	at flash.events::EventDispatcher/dispatchEvent()
	at fl.controls::SelectableList/handleCellRendererClick()

I get that error the second I select a calc and it goes to that frame, and when I input all the information it gives me a "0 and whatever text I had it add" for the answer. Please help, I've been stuck on this for a while.


EDIT: I Believe I did the wrong type of event for lines 22, 25 but I'm still not sure with what type of event I should use. Any advice is greatly appreciated