only call base_class if record isn't nil

Author: Drew-Goddyn

lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb

@@ -3,7 +3,9 @@ module Associations
     class BelongsToPolymorphicAssociation < BelongsToAssociation
       private def replace_keys(record)
         super
-        owner[reflection.foreign_type] = record.class.base_class
+        klass = record.nil? ? record.class : record.class.base_class
+
+        owner[reflection.foreign_type] = klass
       end
     end
   end