WTH ist there no "check config and display errors" button in the file editor add-on

It’s not crucial but a comfort-feature.

Add-on wrap existing software, in the car of the file editor add-on the software used is: “hass-configurator”.

It is a third-party project, which the Home Assistant doesn’t control or manages. We can’t do anything to address this WTH for that reason.

Best place to report this, is with the hass-configurator project.

…/Frenck

Thanks for that hint frenck!
It seems that request already exists and seeks for help: Add "Check Config" link in Configurator settings · Issue #183 · danielperna84/hass-configurator · GitHub