When I drag and drop the Trash from Dolphin to the Folder View Desktop, the following desktop file is created: [Desktop Entry] Icon=unknown Name=trash:/ Type=Link URL[$e]=trash:/ No Trash icon is displayed (empty white paper icon). When I click this file, the message "Malformed URL trash:/" appears. I read something on the mailing list, so this might already be fixed.
Should be fixed by https://cgit.kde.org/kio.git/commit/?id=c600f0cb823e310d772cca383fb5544a33012c3b I didn't change the URL, though, so I wonder why opening doesn't work for you
I have just tested with Dolphin 20.08.1 and cannot reproduce, Trash icon is created on the Desktop and has appropriate icon and opens the Trash folder correctly. If this issue persists for you please reopen this report.