[Error] Failed to call hook 'UpdatePresence' on plugin 'DiscordCore v0.15.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Ext.Discord.DiscordClient.UpdateStatus (Oxide.Ext.Discord.DiscordObjects.Presence presence) [0x0001c] in <618cbb3cd1f24a338ef71fa504209e7a>:0
  at Oxide.Plugins.DiscordCore.UpdatePresence (Oxide.Ext.Discord.DiscordObjects.Presence presence) [0x0000d] in <65eecaca67e943b995e850fb29ae0a1f>:0
  at Oxide.Plugins.DiscordCore.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x003b4] in <65eecaca67e943b995e850fb29ae0a1f>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <008b12f41ec4452da1a5497eeb849299>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <ac41dd3599754d448b8c218b34645820>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <ac41dd3599754d448b8c218b34645820>:0

Unfortunately, this is coming from the Discord Extension and Discord Core has no way to detect if the connection has been broken. Typically Discord Core will notice the timeout and reconnect but there will be these errors until then. They won't hurt anything on the server and can be ignored.