I had never seen this before, anyone know what might be causing this?

(16:14:15) | IPaddress:Port/SteamID/NAME disconnecting: ArgumentException: The Object you want to instantiate is null.
UnityEngine.Object.CheckNullArgument (System.Object arg, System.String message) (at <00000000000000000000000000000000>:0)
UnityEngine.Object.Instantiate[T] (T original) (at <00000000000000000000000000000000>:0)
Planner+Guide.CacheMaterialInstance (UnityEngine.Material asset, UnityEngine.Material& inst) (at <00000000000000000000000000000000>:0)
Planner+Guide.Update (Construction+Target& placement, Construction currentComponent) (at <00000000000000000000000000000000>:0)
Planner.UpdateGuide () (at <00000000000000000000000000000000>:0)
BasePlayer.HeldEntityFrame () (at <00000000000000000000000000000000>:0)
BasePlayer.ClientUpdateLocalPlayer () (at <00000000000000000000000000000000>:0)
BasePlayer.ClientCycle (System.Single deltaTime) (at <00000000000000000000000000000000>:0)
Client.Update () (at <00000000000000000000000000000000>:0)

Version: 42980
(16:17:18) | IPaddress:Port/SteamID/NAME joined [windows/SteamID]
(16:17:18) | NAME[45182767/SteamID] has entered the game
(16:17:39) | IPaddress:Port/SteamID/NAME disconnecting: ArgumentException: The Object you want to instantiate is null.
UnityEngine.Object.CheckNullArgument (System.Object arg, System.String message) (at <00000000000000000000000000000000>:0)
UnityEngine.Object.Instantiate[T] (T original) (at <00000000000000000000000000000000>:0)
Planner+Guide.CacheMaterialInstance (UnityEngine.Material asset, UnityEngine.Material& inst) (at <00000000000000000000000000000000>:0)
Planner+Guide.Update (Construction+Target& placement, Construction currentComponent) (at <00000000000000000000000000000000>:0)
Planner.UpdateGuide () (at <00000000000000000000000000000000>:0)
BasePlayer.HeldEntityFrame () (at <00000000000000000000000000000000>:0)
BasePlayer.ClientUpdateLocalPlayer () (at <00000000000000000000000000000000>:0)
BasePlayer.ClientCycle (System.Single deltaTime) (at <00000000000000000000000000000000>:0)
Client.Update () (at <00000000000000000000000000000000>:0)

Version: 42980