Fix error when opening preferences

Sergio Costasrequested to merge
sergio-costas/desktop-icons-ng:fix-error-when-opening-preferences into master

When opening preferences from gnome-extensions-app, in Gnome 45 and previous it is left in a wrong status, and after closing the preferences, they won't open again. In Gnome 46, an error window will also popup.

This patch fixes it.

Fix #314 (closed)
Fix https://bugs.launchpad.net/ubuntu/+source/gnome-shell-extension-desktop-icons-ng/+bug/2061070

Edited by Sergio Costas

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