I've just noticed this problem on my freshly built arch laptop running plasma 5.9.2. I installed opera and then dolphin and when I open the plasma wastebin widget it launches opera. I have tried creating a new user and the problem is replicated so the error must be global. I tried removing Opera and clicking open on the wastebin widget and this message is now present in journalctl: kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop Unable to detect a web browser to launch 'trash:/'
If you run "xdg-open trash:/" what happens? Can you set the status back to unconfirmed when you reply.
It turns out I was missing the xdg-utils package which contains xdg-open. All working ok now. I'll set the status to RESOLVED.