If you are upgrading joomla 3.7.5 to 3.8.* and you are using Jom Classifieds 3.4.0 you will run into a snag while upgrading Joomla. You will get the error
"Error: 0 Call to undefined method JadminCssMenu::addChild"
1. BEFORE upgrading, go to Extensions > Modules > Manage > Manage.
2. DISABLE Administrator Jomclassifieds Menu
3. Proceed with update and leave it off until you update to Jom Classifieds 3.5.0, tho i have no idea if they fixed this issue as the code is really poorly written and the updates scarce.
And of course, back up your stuff etc etc, but you know that already.