You're on the right track. Ok, bear with me if there are typos, still at work.
lua Code:
function myAddon:ADDON_LOADED(self, event, ...)
-- are there existing vars after everything is loaded? If not, create them
GMovableFrames = GMovableFrames or {} -- put your defaults in the braces
self:UnregisterEvent(event)
end
This is the simplest way to check your vars.