TypeError 1009 won't go away

[KingsGambit]

I know that this is a common error that has been addressed but I've read all the threads and Adobe's library explanation of the error--
but I still cannot figure out what's wrong with my code (it worked earlier).
EDIT: So I think my debugger is saying my problem starts in the addEventListener line:

//this is the document class (MainCharacter.as), inside the Main Character class
		public function MainCharacter()
		{
			init();
		}
		public function init():void
		{
		        stage.addEventListener(KeyboardEvent.KEY_DOWN,ifKeyD);
			stage.addEventListener(KeyboardEvent.KEY_UP,ifKeyU);
			addEventListener(Event.ENTER_FRAME,ifEnterFrame);
		}

Output message:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at MainCharacter/init()
	at MainCharacter()

[[afz]snickelfitz]

If you are adding this to another class, the reference to the stage in the init method is probably causing the error.
The stage is not a valid reference until the class instance is actually on the displaylist.

When you do this:

var mainChar:MainCharacter = new MainCharacter();

The constructor is immediately called and a reference to the stage is made in the init() method, resulting in the error.

The stage is not available until you do this:

addChild(mainChar);

The solution is listen for Event.ADDED_TO_STAGE in the MainCharacter class. This will trigger the Event handler when you call addChild(mainChar);
Make the reference to the stage in the Event handler.

[KingsGambit]

(edit) ah sorry, it's a doc class for the fla. I've read in other threads but none of them have the same problem as mine.

(edit2) It's actually because I had this object on stage with a code written for it (a health bar); I think the problem was that I didn't have an instance name for the health bar movieclip.

Thanks for the help though. I'm just following a beginner tutorial right now; I was curious how to call other classes with my document class.