Don't let items types be null.

Better fail-fast if registry doesn't load? or why-ever else this happens

worldedit-core/src/main/java/com/sk89q/worldedit/blocks/BaseItem.java

@@ -25,6 +25,8 @@ import com.sk89q.worldedit.world.item.ItemType;
 
 import javax.annotation.Nullable;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * Represents an item, without an amount value. See {@link BaseItemStack}
  * for an instance with stack amount information.
@@ -43,6 +45,7 @@ public class BaseItem implements NbtValued {
      * @param itemType Type of the item
      */
     public BaseItem(ItemType itemType) {
+        checkNotNull(itemType);
         this.itemType = itemType;
     }
 
@@ -52,7 +55,8 @@ public class BaseItem implements NbtValued {
      * @param itemType Type of the item
      * @param tag NBT Compound tag
      */
-    public BaseItem(ItemType itemType, CompoundTag tag) {
+    public BaseItem(ItemType itemType, @Nullable CompoundTag tag) {
+        checkNotNull(itemType);
         this.itemType = itemType;
         this.nbtData = tag;
     }