Avoid negative dates using iferror

I have attached a simple example worksheet showing what I am trying to do.

I have a contract start date, a contract finish date, a notice period and a standard review period. To get a date when I should start my contract review I do the following:

(finish date-notice period)- review period.

This is fine when the finish date is populated but if it isn't it results in a negative number.

What I want to achieve is if there is not a finish date then leave the review date cell blank.

Thanks in advance