Steps to reproduce the issue

Install Blog sample data

Log as Super User in frontend.
Stay on the Home page.

In the TOP Author Menu, click on Site Settings
Save the form
Result: {"error":true,"code":0,"message":"Form::loadForm could not load file"}

Also in the TOP Author Menu, click on template settings
Change the description and save (Do not try to change to fluid Layout as it is really broken).
Result: Nothing changes at first. It will change after reloading the page again.
The Page remains stuck on /en/component/config (I test on a multilingual site) and one has to click on Home in the Main menu to get out of there.

Now edit a module and Save it. No need to change anything.
Result: {"error":true,"code":0,"message":"Call to a member function getType() on null"}