see #8740 - fix NPE in core with plugins defining invalid preferences (scripting)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5973 0c6e7542-c601-0410-84e7-c038aed88b3b
fix #8736 - internal preset: "editable=" missing in documentation section

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5974 0c6e7542-c601-0410-84e7-c038aed88b3b