Seems to have Problems with FSE based themes

I have a lot of widgets I created for an intranet site. I wanted to try converting the current child theme to FSE. I loaded this plugin but it doesn’t seem to work on fse themes. When entering the new editor it gets an error message header already sent. If you add the block to the template and save. You start getting errors editor failed attempt recover. There is also a button to access the customizer which has been hidden in FSE. It will bring the hidden customizer up but doesn’t show any widgets to add. Doesn’t work on FSE causes too many errors.