Empty error on backend

After setting up all the needed functions, it looks like it’s working, but I’m missing things like the widget(s). When the plugin is enabled, I see an error area on top of the plugin page (https://prntscr.com/o0ulyb), which is empty, even when I inspect the element with the developer tool of my browser.

Anyone who has an idea?

The page I need help with: [log in to see the link]