Fyi I've seen this error thrown a few times recently.
I do not see any errors thrown around close to it.

(10:08:28) | Failed to call hook 'Discord_MemberRemoved' on plugin 'DiscordCore v0.15.8' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.DiscordCore.Discord_MemberRemoved (Oxide.Ext.Discord.DiscordObjects.GuildMember member) [0x00000] in <535d2d2bc56c4450b1c7a19192c0f7c8>:0 
  at Oxide.Plugins.DiscordCore.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00664] in <535d2d2bc56c4450b1c7a19192c0f7c8>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d09a1f46ca2f4432811bcfe45ad13c7b>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <cf88a28c7fb44d36890d85a78331cc9d>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <cf88a28c7fb44d36890d85a78331cc9d>:0​

The issue should be fixed in version 0.15.9

Thanks for the quick turnaround!