This most probably means that you declared a variable but did not instantiate it or did not assigned value to it and then try to refer to a property of this variable.
In any case, this kind of questions should be accompanied by code and full error stack text.
I would assume this could be that you are utilizing a method that is native to AS3, but not necessarly available to Flash Lite.
I recommend downloading the Community Help Air app. http://www.adobe.com/community/publishing/download.html top left blue box, and then check to see if you are trying to use a feature that is only available to as3 language.
There are toggles that you can turn on and off for FlashLite