Fixed a bug throwing an exception when mapping an interface to a column using UserType

Author: mouk

src/FluentNHibernate/Mapping/Member.cs

@@ -490,7 +490,7 @@ public static IEnumerable<Member> GetInstanceMembers(this Type type)
             type.GetInstanceFields().Each(x => members.Add(x));
             type.GetInstanceMethods().Each(x => members.Add(x));
 
-            if (type.BaseType != typeof(object))
+            if (type.BaseType != null && type.BaseType != typeof(object))
                 type.BaseType.GetInstanceMembers().Each(x => members.Add(x));
 
             return members;