fix #8750 - NPE in ValidatorDialog

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5971 0c6e7542-c601-0410-84e7-c038aed88b3b
see #8749 - use the automatic role suggestions when adding selection to a relation from context menu

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5972 0c6e7542-c601-0410-84e7-c038aed88b3b