Hello guys does anyone know why I get this error when I use the workbench for unlocking item or buying in vending machine.

NullReferenceException: Object reference not set to an instance of an object.
Disconnected (RPC Error in CLIENT_ReceiveSellOrders: Object reference not set to an instance of an object.
  at VendingMachine.OnRpcMessage (BasePlayer player, System.UInt32 rpc, Network.Message msg) [0x00000] in <00000000000000000000000000000000>:0 
  at BaseEntity.CL_RPCMessage (System.UInt32 nameID, System.UInt64 sourceConnection, Network.Message message) [0x00000] in <00000000000000000000000000000000>:0 
  at Client.OnRPCMessage (Network.Message packet) [0x00000] in <00000000000000000000000000000000>:0 
  at Client.OnNetworkMessage (Network.Message packet) [0x00000] in <00000000000000000000000000000000>:0 
  at Facepunch.Network.Raknet.Client.HandleMessage () [0x00000] in <00000000000000000000000000000000>:0 
  at Facepunch.Network.Raknet.Client.Cycle () [0x00000] in <00000000000000000000000000000000>:0 
  at Client.Update () [0x00000] in <00000000000000000000000000000000>:0 ) - returning to main menu​

And this error after reconnect, I can't spawn.

NullReferenceException: Object reference not set to an instance of an object.
Disconnected (Object reference not set to an instance of an object.
  at TechTreeWidget.UnlockPunch (System.Single scale) [0x00000] in <00000000000000000000000000000000>:0 
  at TechTreeDialog.OnInventoryChanged () [0x00000] in <00000000000000000000000000000000>:0 
  at GlobalMessages.OnInventoryChanged () [0x00000] in <00000000000000000000000000000000>:0 
  at BasePlayer.InitLocalPlayer () [0x00000] in <00000000000000000000000000000000>:0 
  at BaseNetworkable.ClientSpawn (ProtoBuf.Entity info) [0x00000] in <00000000000000000000000000000000>:0 
  at Client.CreateOrUpdateEntity (ProtoBuf.Entity info, System.Int64 size) [0x00000] in <00000000000000000000000000000000>:0 
  at Client.OnEntities (Network.Message packet) [0x00000] in <00000000000000000000000000000000>:0 
  at Client.OnNetworkMessage (Network.Message packet) [0x00000] in <00000000000000000000000000000000>:0 
  at Facepunch.Network.Raknet.Client.HandleMessage () [0x00000] in <00000000000000000000000000000000>:0 
  at Facepunch.Network.Raknet.Client.Cycle () [0x00000] in <00000000000000000000000000000000>:0 
  at Client.Update () [0x00000] in <00000000000000000000000000000000>:0 ) - returning to main menu

The Vehicle Airdrops plugin has a known issue which can cause the RPC error if you have a workbench open while the plugin's modified vending machine refreshed its stock nearby. 

Yes the problem is fixed after removing VehicleAirdrops.cs

Thanks @WhiteThunder 🤜🤛