Deriving DateTime Columns with Null Values

Hi Team,

I have issue with the derived column for datetime.
Input is from txt file in yyyymmdd format and deriving the column as
(DT_DATE)(SUBSTRING([Column 13],1,4) + "-" + SUBSTRING([Column 13],5,2) + "-" + SUBSTRING([Column 13],7,2))
it is failing because the column 13 has null how to handle this?

Thanks

Supreeth