Invalid desktop files in XUbuntu 18.04

[Tom Troughton]

This seems similar to https://bugs.launchpad.net/ubuntu/+source/xubuntu-default-settings/+bug/1754888

4 files affected: /usr/share/applications/org.kde.mobile.okular_chm.desktop, /usr/share/applications/org.kde.mobile.okular_djvu.desktop, /usr/share/applications/org.kde.mobile.okular_epub.desktop, /usr/share/applications/org.kde.mobile.okular_tiff.desktop

The message is <Unknown error. Desktop file appears to be valid.> There are Okular desktop files for these mimetypes that work, e.g. /usr/share/applications/okularApplication_chm.desktop

Both broken and working files have the same timestamp.

The broken files e.g. start with <MimeType=image/tiff;
Name=Reader>,
but the working files start with <MimeType=image/tiff;
Terminal=false
Name=Okular>

The broken files have the line <InitialPreference=1>, the broken <InitialPreference=2>