ComboBox Problem

I'm trying to add some item to a combo box but I get the error message below:

//My script:

MyCombo.addItem({ label: "MyItem1"});

MyCombo.addItem({ label: "MyItem2"});

Why?

Any hint?

Thank you.

//the error message:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

  at fl.containers::BaseScrollPane/drawBackground()

  at fl.controls::List/draw()

  at fl.core::UIComponent/drawNow()

  at fl.controls::List/scrollToIndex()

  at fl.controls::SelectableList/scrollToSelected()

  at fl.controls::ComboBox/open()

  at fl.controls::ComboBox/onToggleListVisibility()

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.

  at flash.display::DisplayObjectContainer/removeChild()

  at fl.controls::ComboBox/close()

  at fl.controls::ComboBox/onToggleListVisibility()