+ if (context==null) {
+ if (nullProvider == null)
+ {
+ if (instances != null)
+ {
+ throw new Error(
+ "Implementation error. Structure selection manager's context is 'null'",
+ new NullPointerException("SSM context is null"));
+ }
+ else
+ {
+ nullProvider = new StructureSelectionManager();
+ }
+ return nullProvider;
+ }