RSS Feed Widget button

That’s actually normal. An RSS feed is nothing more than an XML stream of data meant to be viewed by a feed reader, not a browser. Basically, if someone is actually looking for your RSS feed, they already know what to do with it. 🙂

See https://codex.ww.wp.xz.cn/WordPress_Feeds#Feed_Errors_in_Browsers for more details.

I don’t know, where did you get this widget from?

Oh that widget, no that’s unfortunately not possible to directly edit.

You could edit the core file directly, yes, but we don’t support that here, as it could break something else and will always be undone with each update.