Set window.app as default value for button group option "app".

lib/pixi/buttongroup.js

@@ -45,7 +45,7 @@ export default class ButtonGroup extends PIXI.Container {
      * @param {number} [opts.maxWidth] - The maximum width of the button group. If the buttons are wider than the maximum width, the buttons get stacked. Note: The buttons can only be stacked if margin is not zero. Note 2: Load the Greensock ThrowPropsPlugin for smoother animations.
      * @param {number} [opts.maxHeight] - The maximum height of the button group. If the buttons are higher than the maximum height, the buttons get stacked. Note: The buttons can only be stacked if margin is not zero. Note 2: Load the Greensock ThrowPropsPlugin for smoother animations.
      * @param {number} [opts.stackPadding=10] - The padding for stacked buttons.
-     * @param {PIXI.Application} [opts.app] - The PixiJS Application. Must be set if you want to use the mousewheel to scroll your button group. Only used when the buttons are stacked (with maxWidth or maxHeight).
+     * @param {PIXI.Application} [opts.app=window.app] - The PixiJS Application. Must be set if you want to use the mousewheel to scroll your button group. Only used when the buttons are stacked (with maxWidth or maxHeight).
      * @param {number} [opts.padding=Theme.padding] - Button: The inner spacing (distance from icon and/or label) the the border.
      * @param {number} [opts.margin=Theme.margin] - The outer spacing (distance from one button to the previous/next button).
      * @param {string} [opts.iconPosition=left] - Button: The position of the icon in relation to the label. Can be left or right.
@@ -97,7 +97,7 @@ export default class ButtonGroup extends PIXI.Container {
                 maxWidth: null,
                 maxHeight: null,
                 stackPadding: 10,
-                app: null,
+                app: window.app,
                 padding: theme.padding,
                 margin: theme.margin,
                 iconPosition: 'left', // left, right