Comment 1 for bug 1321714

- the problem is that the mathematical connection is deleted two times:

first - it is deleted from DefaultMathNodeRemover because there are no more references to it
(after deletion its parent becomes null, it is no longer possible to determine its container)

second - it is deleted because the math Node is referring to it in the NodeContextTracker (Math node is deleted, hence the related connection must be deleted as well)

current solution: instead of creating an exception about a parent not being a container, ignore deleting node with parent == null