The Help button (button right of kcontrol module) does nothing if khelpcenter is not installed. It should either not be displayed or offer to install khelpcenter using packagekit (same as gwenview offers to install kipi-plugins)
*** Bug 385783 has been marked as a duplicate of this bug. ***
I think that 'kdehelpcenter' must be installed by default with Plasma. This problem happened to me in KDE Neon user edition.
plasma 5.12 beta is affected on Arch Linux.
*** This bug has been marked as a duplicate of bug 405647 ***