Mapping/Map.cs: Give a helpful message if user doesn't specify collection-type for a sorted generic map, instead of NullReferenceException.

Author: oskarb

src/NHibernate/Mapping/Map.cs

@@ -31,12 +31,12 @@ public override CollectionType CollectionType
 				if (IsGeneric && IsSorted)
 				{
 					CheckGenericArgumentsLength(2);
-					if (TypeName.Contains("sorted-list"))
+					if (TypeName != null && TypeName.Contains("sorted-list"))
 					{
 						return
 							TypeFactory.GenericSortedList(Role, ReferencedPropertyName, Comparer, GenericArguments[0], GenericArguments[1]);
 					}
-					else if (TypeName.Contains("sorted-dictionary"))
+					else if (TypeName != null && TypeName.Contains("sorted-dictionary"))
 					{
 						return
 							TypeFactory.GenericSortedDictionary(Role, ReferencedPropertyName, Comparer, GenericArguments[0],
@@ -45,7 +45,7 @@ public override CollectionType CollectionType
 					else
 					{
 						throw new MappingException(
-							"Use collection-type='sorted-list/sorted-dictionary' to choose implementation for generic map");
+							"Use collection-type='sorted-list/sorted-dictionary' to choose implementation for generic map owned by " + this.OwnerEntityName);
 					}
 				}
 				return base.CollectionType;