ComboBox Used to Change Frames

I have a couple of programs where a ComboBox is used to change frames.  Mostly it works okay but if the ComboBox is held open and the mouse pointer is moved back and forwards repeatedly over the options (sort of thing you do when Debugging) I get the error:

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

  at fl.controls::ComboBox/close()

  at fl.controls::ComboBox/onListItemUp()

This only happens on ComboBoxes where the options are used to change frames.  An example of the code i use is below

CalcCombo.addEventListener(Event.CHANGE, CalcSelectPage);

function CalcSelectPage (event:Event):void

{

  if (CalcCombo.selectedItem.label == "FLOW") {

  gotoAndStop(3);

  }

  else if (CalcCombo.selectedItem.label == "EFFICIENCY") {

  gotoAndStop(4);

  }

  else if (CalcCombo.selectedItem.label == "POWER") {

  gotoAndStop(2);

  }

}

Does anybody know whats going on?