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Why is subclassing goog.ui.Button producing this error?
/**
 * @fileoverview This button makes a new widget.
 * @author David Faux
 */

goog.provide('app.ui.NewWidgetButton');

goog.require('goog.ui.Button');


/**
 * Button for creating a new widget.
 * @constructor
 * @param {goog.ui.ButtonRenderer=} opt_renderer Optional renderer used to
 *    render or decorate the button.
 * @param {goog.dom.DomHelper=} opt_domHelper Optional DOM hepler, used for
 *    document interaction.
 * @extends {goog.ui.Button}
 */
app.ui.NewWidgetButton = function(opt_renderer, opt_domHelper) {
  goog.base(
    this,
    /** @type {goog.ui.ControlContent} */ 'New Widget',
    opt_renderer,
    opt_domHelper);
}
goog.inherits(app.ui.NewWidgetButton, goog.ui.Button);

`Uncaught TypeError: Object #<Object> has no method 'setContentInternal'`

var newButton = new app.ui.NewWidgetButton();