fix issue56: "CADTools - Drawing Blocks throws NPEs" (again)

see openjump-gis/openjump#56

[INFO] cadtools-extension-2.1.4.jar - MD5 : 8222a488adacb343fd2ab93cf5ee96ab
[INFO] cadtools-extension-2.1.4.jar - SHA-1 : 805efb2f0c035351fd71fe3331274eb4ae145094