The NEWID() function is not known for certain by the compiler to always return a value (even though you and I know that), so the only way to fix this is to use ISNULL again
ISNULL(ISNULL(Anchor.[Guid], NEWID()), CAST(0x0 AS uniqueidentifier))
The last parameter never actually gets executed.
Obviously, if you don't actually need NEWID() then you can just use ...