How do you check if the user hasn't made a selection on a combo box?

[RansomHostage]

Just as the title says really! How do you check if the user hasn't made a selection on a combo box? Here's what I've tried, to no avail:

tShirtBuy.addEventListener(MouseEvent.CLICK, sendShirtToCheckout);

function sendShirtToCheckout (evt:MouseEvent):void {
	if (tShirtColour.selectedItem.value == "" || tShirtSize.selectedItem.value == "") {
		trace("worked!")
	} else {
		trace("fail!")
	}
}

which gives the error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at indoorshredwebsite_fla::shopmc_17/sendShirtToCheckout()