Why can't I use the "id" attribute on <button> elements in layouts xml?

I'm using <button> elements to handle input -- they are supposed to inherit from Selectables -> Drawables, which support the id attribute. When I add an id attribute to the <button> (to be able to look it up with View.findDrawableById()) I get this error in terminal:

ERROR: edge1030plus: ...\widget\bin\gen\006-B3570-00\source\Rez.mcgen:40,12: No class found for new object instantiation.

<button id="play" x="20" y="130" width="111" height="111" behavior="onPlayButton">
    <state id="stateDefault" bitmap="@Drawables.DefaultPlayButton" />
    <state id="stateHighlighted" bitmap="@Drawables.HighlightedPlayButton" />
    <state id="stateSelected" bitmap="@Drawables.SelectedPlayButton" />
    <state id="stateDisabled" color="Graphics.COLOR_BLACK" />
</button>

I am able to get a reference to it via BehaviorDelegate with code like this, but that only happens after an interaction has occurred. Is there another way to access the element from the view itself?

class MyBehaviorDelegate extends Ui.BehaviorDelegate {

    function onSelectable(event) {
        event.getInstance().setVisible(false);
    }

}