Null Length Error on Craft Version 3
The error tells you that faqs is null. As you're using the first() function to execute your element query, this means that your query has no matches.

Are you sure that the section handle (FAQ) is in uppercase?
The correct way to create an entry query in Craft 3 is craft.entries(). Although it may work without the parenthesis, it can't hurt to add them.
You can replace .search('slug::' ~ entry.slug)  with .slug(entry.slug). It's easier to understand and should have the same effect.
Are you sure that the query always returned something in Craft 2? It may be worth adding a null check to avoid the error: {% if faqs and faqs.questionsanswers and faqs.questionsanswers|length &gt; 0 %}