Mmm,
I see what you mean, but clearly the error you paste is from that plugin.
If you disable it you’ll not have that error, do you have any other error?
Did you try disabling all your plugins?
Consider that the widget section in the customizer (appearance -> customize) is added by wp via js basically at the end of the customizer settings loading (after other js), so if something (js/php) error occurs .. that section might be not loaded (it depends on where the break occurs).
It might also be a theme problem, or a plugin conflicting issue with the theme (in this case it’s not necessary a theme problem :D).
Widget section works fine here.
So the only thing you can do is
– disable all your plugins and see if it still happens
– switch to a standard wp theme (like twentyfifteen) and see if it still happens.
Another thing.. this message:
“Der genereres og vises i ?jeblikket ingen widgetomr?der i forh?ndsvisningen. Navigér i forh?ndsvisningen til en skabelon, der bruger et widgetomr?de, for at f? adgang til dens widgets her.”
Does the page you see in the preview have any widget area? A right/left sidebar, the footer.
You use a maintenance plugin, do you display it in the preview?
Consider that the widget section is “contextualized” if you don’t have any widget area in the preview page it will display that message, this doesn’t mean there’s something wrong, you just need to access the customizer from a page which has widget areas, or navigate through them in the preview.
Hope this helps.
