Comment 54 for bug 133133

Ideally dolphin.desktop is inside /usr/share/applications/kde4/ , so you have to prefix the entry with kde4-, so it becomes:
x-directory/normal=kde4-dolphin.desktop

Check where dolphin.desktop is located, I don't use ubuntu so I don't know where it's exactly on your system.

Another thing I noticed is I have an entry named "file" in Edit> preferences> applications, selecting "use other" and then selecting /usr/bin/xdg-open, makes it open in the DE's preferred file manager. On a fresh profile I don't see the "file" entry, I don't know what creates it...