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. Does anyone have any idea how to reset this so it launches with dolphin as expected?

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:/'

It turns out I was missing the xdg-utils package which contains xdg-open. All working ok now.