MT#7941 Fix handling empty destination set in pref

lib/NGCP/Panel/Controller/Subscriber.pm

@@ -664,11 +664,14 @@ sub preferences :Chained('base') :PathPart('preferences') :Args(0) {
             ->search({ type => $type });
         $cfs->{$type} = [];
         foreach my $map($maps->all) {
-            my @dset = map { { $_->get_columns } } $map->destination_set->voip_cf_destinations->search({},
+            my @dset = ();
+            if($map->destination_set) {
+                @dset = map { { $_->get_columns } } $map->destination_set->voip_cf_destinations->search({},
                     { order_by => { -asc => 'priority' }})->all;
                 foreach my $d(@dset) {
                     $d->{as_string} = NGCP::Panel::Utils::Subscriber::destination_as_string($d);
                 }
+            }
             my @tset = ();
             my $tset_name = undef;
             if($map->time_set) {