If you upgrade to Plasma 5.18 with the widgets already locked in a previous version (in my case 5.17.5) you are unable to use the Customize Layout option and no widget or panel options are shown when right clicking the desktop or panels.
STEPS TO REPRODUCE
1. Be on a Plasma release prior to 5.18
2. Lock the widgets
3. Upgrade to 5.18
The Customize Layout option is greyed out in the right click menu of the desktop. No widget or panel options are shown when either right clicking the desktop or panels.
The ability to use these features are expected (i.e. they are available to use).
Operating System: Kubuntu 19.10
KDE Plasma Version: 5.18.0
KDE Frameworks Version: 5.67.0
Qt Version: 5.12.4
Kernel Version: 5.3.0-29-generic
OS Type: 64-bit
Processors: 12 × Intel® Core™ i7-8750H CPU @ 2.20GHz
Memory: 15.4 GiB of RAM
I assume the most graceful way to handle this is to check to see if widgets are locked when upgrading to 5.18 and unlock them. I was able to do this from the command line using the following command.
qdbus org.kde.plasmashell /PlasmaShell evaluateScript "lockCorona(false)"
*** This bug has been marked as a duplicate of bug 417424 ***