ValueError, when exporting an armature, that is not in the root, with collapse armatures enabled

[StjerneIdioten]

In LXML elements can only have one parent, where in Element Tree they can belong to multiple. The exporter manually deletes elements after moving them to a different parent during skinned mesh export (when collapsing armature nodes), but if using LXML this will crash since it has already been moved by LXML itself. This will only happen if the armature to be exported is not a root node itself.