This can be tricky to help with without some more details.
Sounds like PHP 7 has run into some compatibility errors.
Seems that you need to look at line 87 of /homepages/36/d129703868/htdocs/mightyoak/bhd-wp/wp-content/themes/construct/lib/classes/menu-walker.php
Is this the Construct Theme and are you running the latest version?