Error while studying the subject of the study table

Failed to call hook 'OnServerSave' on plugin 'XPerience v1.4.0' (KeyNotFoundException: The given key was not present in the dictionary.)
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <eae584ce26bc40229c1b1aa476bfa589>:0

  at Oxide.Plugins.XPerience.OnServerSave () [0x00050] in <82ec9fe0afd649618dcefaf2c0a8e6b9>:0

  at Oxide.Plugins.XPerience.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x01973] in <82ec9fe0afd649618dcefaf2c0a8e6b9>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <dfcb48ea05694263bbc08e62a39c274c>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <dfcb48ea05694263bbc08e62a39c274c>:0

This is a save issue not a study table issue which i believe your refering to the research table.. I have have a few reports of this error coming up randomly and working to track it down.