2.0 nightly build #434.

Steps to reproduce:
1. Add a root container then add a Chart, for example, BarChar.
2. Select NumberAxis, then try to delete it by pressing "DEL" or right click and select "delete" on the context menu.
There is no responding, and the context menu doesn't close.
3. Now, if trying to add other component into the root container, SceneBuilder crashes.