I was curious what selecting "Make Default" did in relation to Flatpak vs Neon sources in the Discover settings, and went to the Help menu item to find out. It opens a web browser to https://docs.kde.org/index.php?branch=stable5&language=en_US&application=discover&path=/ and it states "The application discover could not be found. Please check if you entered the name correctly, or use the navigation on the left to access the complete KDE User Documentation."
I can confirm this on KDE Neon User 5.12.1 with the same result as Andrew. To be clear, this is about the feature in Settings → Help...→Discover Handbook (or Settings→press F1).
Discover does not seem to have a docs.kde.org entry.
May I dare suggest that the link to the Handbook is removed from Discover Settings until a Discover handbook page has been created? Or maybe it will be sufficient to link to the Userbase entry instead; https://userbase.kde.org/Discover
Git commit 4153dc99d2e9749fcfc0652575d9818171fb9f80 by Aleix Pol.
Committed on 22/02/2018 at 17:28.
Pushed by apol into branch 'Plasma/5.12'.
Remove handbook menu entry
There is no handbook
M +0 -10 discover/DiscoverObject.cpp
M +0 -1 discover/DiscoverObject.h
M +0 -1 discover/qml/SourcesPage.qml