Just ran into the same issue, but the solution above didn’t help.
Blank nodejs project with BS5, tried to use a bootswatch theme and the page doesn’t render in design view. The layout file seems ok, therefore (I assume) looks ok in browser, but the page doesn’t render in wappler.
Not sure it is a bug or a feature, but the BS5 bootswatch themes are missing under Project settings/Frameworks. (added BS5 local instead, then added the bootswatch theme to the layout manually)
Tried to add the bootswatch theme manually to the content page, it is available, can see in code view that it will be included in the layout file, but when I save the file it disappears from code view?
The question is the same, what am I doing wrong? 