Right now the bindings just pipe to gtk_tree_store_iter_is_valid:

public boolean isIterValid(TreeIter iter) {
return gtk_tree_store_iter_is_valid(getHandle(), iter.getHandle());
}

However if iter is null, iter.getHandle() will raise an NPE. That requires me to
either:

1) Handle the possible NPE whenever I do an TreeStore.isIterValid()

or 

2) Test for null myself.

Everytime I need to check an TreeIter, right now that requires me to write two
tests. I request that isIterValid do the null test, and return false if null.
(After all I don't care why the TreeIter is invalid, whether it points to
nowhere or is a null, just that it is invalid)