SPIGOT-6506: Fix crash with custom inventories

nms-patches/net/minecraft/world/inventory/Container.patch

@@ -21,6 +21,15 @@
  public abstract class Container {
  
      public static final int SLOT_CLICKED_OUTSIDE = -999;
+@@ -43,7 +57,7 @@
+     public NonNullList<Slot> slots = NonNullList.a();
+     private final List<ContainerProperty> dataSlots = Lists.newArrayList();
+     private ItemStack carried;
+-    private final NonNullList<ItemStack> remoteSlots;
++    public NonNullList<ItemStack> remoteSlots;
+     private final IntList remoteDataSlots;
+     private ItemStack remoteCarried;
+     @Nullable
 @@ -57,6 +71,27 @@
      private ContainerSynchronizer synchronizer;
      private boolean suppressRemoteUpdates;

src/main/java/org/bukkit/craftbukkit/inventory/CraftContainer.java

@@ -223,6 +223,7 @@ public class CraftContainer extends Container {
         if (delegate != null) {
             this.lastSlots = delegate.lastSlots;
             this.slots = delegate.slots;
+            this.remoteSlots = delegate.remoteSlots;
         }
 
         // SPIGOT-4598 - we should still delegate the shift click handler