Go ahead and chose “chromium” as the default browser in the “settings” application, and hit apply. That setting will likely stick, but Chrome will not be the default browser anyway. A bug in KDE Plasma prevents this, but you can drill down deeper into the configuration information and make it work:
Go to your settings tool.
Go to Configure File Associations
Filter the list for “html”
Find the entry for xhtml+xml and move “Chromium” to the top of the list.
Do the same for “html”
Your problem is probably now solved.