OO-559: fix RS if a use try to open the catalog at a deleted catalog entry

src/main/java/org/olat/catalog/ui/CatalogController.java

@@ -1197,6 +1197,7 @@ public class CatalogController extends BasicController implements Activateable2
 		Long catId = catCe.getOLATResourceable().getResourceableId();
 		if(catId == null || catId.longValue() == 0l) return;//nothing to do
 		CatalogEntry ce = CatalogManager.getInstance().loadCatalogEntry(catId);
+		if(ce == null) return;//catalog entry not found, do nothing
 		switch(ce.getType()) {
 			case CatalogEntry.TYPE_NODE: {
 				reloadHistoryStack(ureq, catId);