twod.PolygonsSet.getSize produces NullPointerException if BSPTree has no nodes

org.apache.commons.math3.geometry.euclidean.twod.PolygonsSet.getSize() uses a tree internally:

final BSPTree<Euclidean2D> tree = getTree(false);

However, if that tree contains no data, it seems that the reference returned is null, which causes a subsequent NullPointerException.

Probably an exception with a message ("tree has no data") would clarify that this is an API usage error.