Currently getting this error was working before but all of a sudden stopped working. 

07:04 [Error] Failed to call hook 'OnUserChat' on plugin 'UFilter v5.0.7' (InvalidCastException: Null object cannot be converted to a value type.)
  at System.Convert.ChangeType (System.Object value, System.Type conversionType, System.IFormatProvider provider) [0x00029] in <437ba245d8404784b9fbab9b439ac908>:0 
  at System.Convert.ChangeType (System.Object value, System.Type conversionType) [0x0000c] in <437ba245d8404784b9fbab9b439ac908>:0 
  at Oxide.Core.Plugins.Plugin.Call[T] (System.String hook, System.Object[] args) [0x00008] in <1fe782f4b27a43ae85d29ebb2d56c007>:0 
  at Oxide.Plugins.UFilter.TakeAction (Oxide.Core.Libraries.Covalence.IPlayer player, System.String text, System.String[] list, System.String action, System.String reason) [0x00136] in <f73b1d6e29bc45df8b8d5ee354c296cc>:0 
  at Oxide.Plugins.UFilter.ProcessText (System.String text, Oxide.Core.Libraries.Covalence.IPlayer player) [0x000f9] in <f73b1d6e29bc45df8b8d5ee354c296cc>:0 
  at Oxide.Plugins.UFilter.HandleChat (Oxide.Core.Libraries.Covalence.IPlayer player, System.String message) [0x0000b] in <f73b1d6e29bc45df8b8d5ee354c296cc>:0 
  at Oxide.Plugins.UFilter.OnUserChat (Oxide.Core.Libraries.Covalence.IPlayer player, System.String message) [0x0000b] in <f73b1d6e29bc45df8b8d5ee354c296cc>:0 
  at Oxide.Plugins.UFilter.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0047d] in <f73b1d6e29bc45df8b8d5ee354c296cc>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <8a646463979e4a1d9c0e2f1298e9483f>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <1fe782f4b27a43ae85d29ebb2d56c007>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <1fe782f4b27a43ae85d29ebb2d56c007>:0 ​

Are you using the Slap plugin?

nope and its not enabled

for some reason  changing onuserchat to onplayerchat fixed it

Changing OnUserChat to OnPlayerChat would cause it to not be called. ;)

idk it went from not working to working. I had to just output the call to the other hook to change it to a iplayer. if that's what your talking about.

Well, that's not really a solution... and if that fixed your issue, that's an indication of an underlying issue with your server data in regards to Oxide and the universal API.

Are you using Better Chat?

No we are not

Could you provide your configuration?