I have a text field mainLoadText placed on the stage at authoring time. How do I access it, from the timeline?

If I use stage.mainLoadText I get

1119: Access of possibly undefined property mainLoadText through a reference with static type Class.

and If I use just mainLoadText I get

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Package:ocument/Package::frame1()