[Discord Extension] An error has occured: Response: An exception has occurred during an OnMessage event.
[Discord Core] WebSocketError: System.ArgumentNullException: Value cannot be null.
Parameter name: o
at Newtonsoft.Json.Linq.JToken.FromObjectInternal (System.Object o, Newtonsoft.Json.JsonSerializer jsonSerializer) [0x00038] in <7126fb4f5c5b40029458a4f975f98d13>:0
at Newtonsoft.Json.Linq.JObject.FromObject (System.Object o, Newtonsoft.Json.JsonSerializer jsonSerializer) [0x00000] in <7126fb4f5c5b40029458a4f975f98d13>:0
at Newtonsoft.Json.Linq.JObject.FromObject (System.Object o) [0x00006] in <7126fb4f5c5b40029458a4f975f98d13>:0
at Oxide.Ext.Discord.WebSockets.SocketListner.SocketMessage (System.Object sender, WebSocketSharp.MessageEventArgs e) [0x00fba] in <618cbb3cd1f24a338ef71fa504209e7a>:0
at WebSocketSharp.Ext.Emit[TEventArgs] (System.EventHandler`1[TEventArgs] eventHandler, System.Object sender, TEventArgs e) [0x00009] in <7126fb4f5c5b40029458a4f975f98d13>:0
at WebSocketSharp.WebSocket.messagec (WebSocketSharp.MessageEventArgs e) [0x00003] in <7126fb4f5c5b40029458a4f975f98d13>:0

is this the reason why Discord Rewards bot keeps disconnecting after 2 hours or so?

Discord Rewards doesn't have any relation to Discord Core but the issue with the extension, not the plugin.