Should fix server crash problem when harvesting biomass

Plugin Cubot/src/main/java/net/simon987/cubotplugin/CubotInventory.java

@@ -31,7 +31,7 @@ public class CubotInventory extends CubotHardwareModule {
         super(cubot);
 
         inventory = new HashMap<>();
-        inventory.put(2, new ItemCopper(new Document()));
+        inventory.put(2, new ItemCopper(new Document()));  // TODO: Remove debug value
     }
 
     public CubotInventory(Document document, ControllableUnit cubot) {

Plugin Plant/src/main/java/net/simon987/biomassplugin/ItemBiomass.java

@@ -17,6 +17,10 @@ public class ItemBiomass extends Item {
         return ID;
     }
 
+    public ItemBiomass() {
+        super(null);
+    }
+
     public ItemBiomass(Document document) {
         super(document);
     }

Server/src/main/java/net/simon987/server/game/item/ItemCopper.java

@@ -12,6 +12,10 @@ public class ItemCopper extends Item {
         return ID;
     }
 
+    public ItemCopper() {
+        super(null);
+    }
+
     public ItemCopper(Document document) {
         super(document);
     }

Server/src/main/java/net/simon987/server/game/item/ItemIron.java

@@ -11,6 +11,10 @@ public class ItemIron extends Item {
         return ID;
     }
 
+    public ItemIron() {
+        super(null);
+    }
+
     public ItemIron(Document document) {
         super(document);
     }

Server/src/main/java/net/simon987/server/game/world/TileMap.java

@@ -28,8 +28,6 @@ public class TileMap implements JSONSerialisable, MongoSerializable {
     public static final int VAULT_FLOOR = 4;
     public static final int VAULT_WALL = 5;
 
-    public static final int ITEM_IRON = 3;
-    public static final int ITEM_COPPER = 4;
 
     /**
      * The map of tile