fixed a bug when a matrix listing didn't have a forum attached

docs/upgrades/upgrade_7.5.40-7.6.10.pl

@@ -354,6 +354,7 @@ sub upgradeMatrix {
             # migrate comments
             if($listing->{forumId}){
                 my $forum = WebGUI::Asset::Wobject::Collaboration->new($session, $listing->{forumId});
+                if ( defined $forum ) {
                 my @comments = @{ $forum->getLineage(['descendants'], {
                     includeOnlyClasses  => ["WebGUI::Asset::Post", "WebGUI::Asset::Post::Thread"],
                     returnObjects       => 1,
@@ -367,6 +368,7 @@ sub upgradeMatrix {
                 $newMatrixListing->addComment($content,0,$ownerUser);
                 }
             }
+            }
         }
     }
     $db->write("drop table Matrix_listing");