Currently you have to select the item and click the button "Add" to add the item to the panel. Please add the additional possibility to use a double-click on the item in the treeview to add it to the panel.
Simon Steinbeiss referenced this bugreport in commit 4dd56023c9a736c56fba1cf6884aa360da88e285 Make double-click add plugin in the "Add item" dialog (Bug #9784) https://git.xfce.org/xfce/xfce4-panel/commit?id=4dd56023c9a736c56fba1cf6884aa360da88e285
Only took 5 years, but here you go.
Better late than never, thank you. :)