GoodBoyDigital/pixi.js — pull request #457 by photonstorm (2013-12-23 03:20:16+00:00)
Title: Fix for addChild when parent is null

Previously addChild was checking this: `if(child.parent != undefined)` but testing just for 'undefined' isn't enough, as the parent is set to 'null' in DisplayObject.js, so this condition fails and throws an error as it tries to removeChild from null.