Programmatically created combobox error

Hi, I’m trying to create a combobox inside a package and add it to the stage later on. However, I’m getting the following error when I try to click on the combobox:
Error #1009: Cannot access a property or method of a null object reference.

Here is the package for the combobox:

package {
    import flash.display.*;
    import fl.controls.*;
    public class Combo extends MovieClip {
        private var combo:ComboBox = new ComboBox();
        public function Combo(dropData:XMLList) {
            trace("dropData passed: " +dropData);
            combo.width = 300;
            combo.move(10, 10);
            var i:uint;
            var numItems:int = dropData.drop.length();
            trace("numItems is: " + numItems);

            for (i = 0; i < numItems; i++) {
                var test = dropData.drop*.attribute("label");


And here is what I have on the stage:

    var combo:Combo = new Combo(dropData);

Now, the component is inside my library, and it actually appears when I test the swf, but if I try to click on it, I’m getting the error mentioned above and here in detail:

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/focusOutHandler()

Thanks for your help,