[kmainwindow] Don't delete entries from an invalid kconfiggroup
If for some reason the config group we're trying to delete entries from
doesn't exist, the app will assert in KConfigGroup::deleteEntry in
Q_ASSERT_X(isValid(), "KConfigGroup::deleteEntry", "accessing an invalid group");
So let's guard against that by only deleting the entry from the config group if the group itself is valid. Maybe it's not, for some reason.
BUG: 427236 FIXED-IN: 5.75