Fix NPE

Signed-off-by: Chaoscaot <chaoscaot444@gmail.com>

BauSystem_15/src/de/steamwar/bausystem/features/world/InventoryListener_15.java

@@ -32,12 +32,13 @@ public class InventoryListener_15 {
     public static boolean checkItemStack(ItemStack item) {
         net.minecraft.server.v1_15_R1.ItemStack nmsItem = CraftItemStack.asNMSCopy(item);
         NBTTagCompound tag = nmsItem.getTag();
-        assert tag != null;
+        if (tag != null && tag.hasKey("BlockEntityTag")) {
             NBTTagCompound blockTag = tag.getCompound("BlockEntityTag");
-        assert blockTag != null;
             if (blockTag.hasKey("Items")) {
                 return drillDown(blockTag.getList("Items", 10), 0, 0) > threshold;
             }
+        }
+
         return false;
     }