When adding a Dynamic Category Property to a ZODB Property Sheet, the

expression has not been filled yet, thus re-generated accessors could
have failed


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43208 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Core/DynamicCategoryProperty.py

@@ -55,7 +55,11 @@ class DynamicCategoryProperty(XMLObject):
     """
     Return the filesystem definition of the property
     """
-    return Expression(self.getCategoryExpression())
+    expression_string = self.getCategoryExpression()
+    if expression_string is None:
+      return None
+
+    return Expression(expression_string)
 
   security.declareProtected(Permissions.AccessContentsInformation,
                             'importFromFilesystemDefinition')