Dynamic Item Description Based on Attributes - Derived Property, FOR XML PATH not working -- help?

Greetings!

I am attempting to create a derived field in CSI SLItems that will generate a custom Item description dynamically, based on Item Attributes. Because multiple attributes can exist per Item, I am needing to get columns from multiple rows and flatten them to fit in one row, space delimited.

However, the code below is yielding 'Incorrect syntax near the keyword 'NULL'. Any assistance would be greatly appreciated!! Workaround also welcome. Thank you!

(SELECT
STUFF(
(SELECT ' ' + AttributeValue
FROM AttributeValueView avv
WHERE avv.RefRowPointer = RowPointer AND avv.AttributeValue IS NOT NULL
FOR XML PATH(''), TYPE).value('(./text())[1]', 'nvarchar(max)'),1,1,''))