SchematicSelector.renderItem null material replaced to Barrier

SpigotCore_Main/src/de/steamwar/inventory/SchematicSelector.java

@@ -236,7 +236,7 @@ public class SchematicSelector {
         Material m = SWItem.getMaterial(node.getItem());
 
         if(m == null) {
-            m = Material.AIR;
+            m = Material.BARRIER;
         }
 
         String name = Core.MESSAGE.parse(filter.name == null?"SCHEM_SELECTOR_ITEM_NAME":"SCHEM_SELECTOR_ITEM_NAME_FILTER", player, node.getName());