initXML3DElement() has to check if the element is still a member of the DOM

[afrische]

Consider the following:

1. a xml3d element is added and directly removed, e.g. in a qunit fixture
2. the MutationObserver will fire and eventually initXML3DElement() will be called on the added nodes
3. the element has no longer a parent and an exception will be thrown

hth
Andreas

[csvurt]

Thanks for the report, I'll schedule this for the next hotfix.