Failed to call hook 'OnInventoryNetworkUpdate' on plugin 'ItemRetriever v0.6.2' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Core.Libraries.Permission.UserHasPermission (System.String playerId, System.String permission) [0x0000a] in <157a94ee66ab4a7991faecd1eb84be3b>:0

  at Oxide.Plugins.VirtualItems+Configuration.DetermineBestRuleset (Oxide.Core.Libraries.Permission permission, BasePlayer player) [0x0003e] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.VirtualItems+RulesetManager.Get (BasePlayer player) [0x00018] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.VirtualItems.<RegisterAsItemSupplier>m__3 (BasePlayer player, System.Collections.Generic.List`1[T] saveList) [0x00000] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.ItemRetriever+SupplierManager.SerializeForNetwork (BasePlayer player, ProtoBuf.ItemContainer containerData, System.Int32& nextInvisibleSlot) [0x00039] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.ItemRetriever.SerializeForNetwork (BasePlayer player, ProtoBuf.ItemContainer containerData) [0x00065] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.ItemRetriever.OnInventoryNetworkUpdate (PlayerInventory inventory, ItemContainer container, ProtoBuf.UpdateItemContainer updatedItemContainer, PlayerInventory+Type inventoryType) [0x0001b] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.ItemRetriever.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x001cc] in <efdcd40ee45c43228a41ed88725779df>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <23ba99f131254889867c71f0bd137b1d>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <157a94ee66ab4a7991faecd1eb84be3b>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <157a94ee66ab4a7991faecd1eb84be3b>:0

This error is originating in Virtual Items. Somewhat surprisingly, the error happens when calling the Oxide permissions library. NullReferenceException is only possible in this case if the player has a null steam id. I'm not sure under what circumstances that can happen, perhaps for bots. Are you sure this error occurs for a valid player? If you are seeing Virtual Items not functioning correctly, that may be unrelated to this particular error (but maybe you get other errors) since your player should have a valid steam id attached.