TypeError: Error #2007: パラメータ child は null 以外でなければなりません。 at flash.display::DisplayObjectContainer/addChildAt() at fl.controls::BaseButton/drawBackground() at fl.controls::BaseButton/draw() at fl.core::UIComponent/drawNow() at fl.controls::ScrollBar/draw() at fl.controls::UIScrollBar/draw() at fl.core::UIComponent/drawNow() at fl.controls::TextArea/updateScrollBars() at fl.controls::TextArea/drawLayout() at fl.controls::TextArea/draw()