Unknown mod error, help with identifying

[Daros]

I could use some help in identifying the cause of the following error. It occures when trying to save the game. The save appears to load just fine, despite giving another error upon loading.

Any idea what causes this and how to fix it?
Mod list is quite huge, about 110 mods.

Code Select Expand

Exception registering RimWorld.Bill_Production [excepted] in loaded object directory with unique load ID [excepted]: System.NullReferenceException: Object reference not set to an instance of an object
  at RimWorld.Bill.GetUniqueLoadID () [0x00000] in <filename unknown>:0
  at Verse.LoadedObjectDirectory.RegisterLoaded (ILoadReferenceable reffable) [0x00000] in <filename unknown>:0
Verse.Log:Error(String)
Verse.LoadedObjectDirectory:RegisterLoaded(ILoadReferenceable)
Verse.CrossRefHandler:ResolveAllCrossReferences()
Verse.ScribeLoader:FinalizeLoading()
Verse.Game:LoadGame()
Verse.SavedGameLoader:LoadGameFromSaveFile(String)
Verse.Root_Play:<Start>m__0()
Verse.LongEventHandler:RunEventFromAnotherThread(Action)
Verse.LongEventHandler:<UpdateCurrentAsynchronousEvent>m__1()


[kaptain_kavern]

I recall some error that was only occurring when a trader is present and that was preventing me to save. As soon as the trader was gone, I could save again.

The problem was coming from some wrongly formatted Traders Def in a mod