Paul Speed c73fd99dd6 Fixed the null child check to throw IllegalArgumentException instead of NullPointerException
because user code should never throw NullPointerException.
Also made trying to add a child to itself an error instead of a no-op.  Attempting
to do something like guiNode.attachChild(guiNode) will now throw an IllegalArgumentException.
2019-11-28 15:56:15 -05:00
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