Filling somes blanks here with my own guesses, but basing on what the OP has, I suspect we could do something like this instead:

SELECT *
FROM [dbo].[PROT_TITOLARIO] PT
     CROSS APPLY (VALUES(CASE WHEN PT.IdSottocategoria LIKE '%[^0-9.]%' THEN NULL ELSE PT.IdSottocategoria END)) TC(IdSottocategoria)
     CROSS APPLY (VALUES(CONVERT(hierarchyid,'/'+REPLACE(TC.IdSottocategoria,'.','/')+'/')))V(Hid)
ORDER BY CASE WHEN V.Hid IS NULL THEN 1 ELSE 0 END,
         V.Hid,
         PT.IdSottocategoria;

I use a CASE expression to check that the value of IdSottocategoria only contains numerical values and . 's and then just CONVERT those values. That works for the sample values the OP supplied: db<>fiddle

To reiterate my comments, SQL Server 2008 R2 SP2 is a really old version of SQL Server. SP3 came out some time ago for 2008R2, and SP2 is not patched against the Spectre and Meltdown vulnerabilities. Even if you aren't updating the version of SQL Server for a bit (which should heavily be reconsidered) I cannot more strongly suggest you get that server updated to SP3 GDR . This is especially true if you are in a country that has GDPR (or similar) legislation, as your local authority will see unpatched (and unsupported) software as a huge concern and will not reflect well on the list of preventative measures in the result of a breach.

Could try wrapping it in a try/catch block like this

declare @vDate DATETIME

BEGIN TRY

SELECT @vDate = CONVERT(datetime, '12/31/2010')

END TRY

BEGIN CATCH

SELECT @vDate = null

END CATCH

SELECT @vDate

BEGIN TRY

SELECT @vDate = CONVERT(datetime, 'eggs')

END TRY

BEGIN CATCH

SELECT @vDate = null

END CATCH

SELECT @vDate