Advice for future help requests:
When looking for help and troubleshooting, please show the steps taken so that we know exactly what you’ve done. Saying it didn’t help, is usually not enough information.
In this case, it maybe is enough if you logged into a KDE/Openbox session and tried to click the app launcher, because essentially this is a KDE-specific bug. Using an Openbox-only session would not be able to reproduce this bug, because you wouldn’t be able to click KDE’s app launcher.
Now, a possible solution:
According to https://bugs.kde.org/show_bug.cgi?id=359634, you may need to uninstall then reinstall plasma-framework5.
This is an old bug report, but it may fix the issue.
sudo pacman -Rs plasma-framework5
sudo pacman -S plasma-framework5
If it doesn’t fix it, it’s a good idea to learn how to search for fixes, bug reports, etc. The easiest way is to do a web search with the error message you get.
For other ways, see here: [Wiki] How To Check Bug Reports