Modded Rimworld stops responding when playing for about an hour

[Toaster]

The output_log says this before it crashes :

Exception while generating thing set: System.ArgumentNullException: Argument cannot be null.
Parameter name: type
at System.Activator.CheckType (System.Type) <0x00095>
at System.Activator.CreateInstance (System.Type,bool) <0x00027>
at System.Activator.CreateInstance (System.Type) <0x0001e>
at Verse.ThingMaker.MakeThing (Verse.ThingDef,Verse.ThingDef) <0x0035e>
at RimWorld.StockGeneratorUtility.TryMakeForStockSingle (Verse.ThingDef,int) <0x0021e>
at RimWorld.StockGeneratorUtility/<TryMakeForStock>c__Iterator0.MoveNext () <0x000eb>
at RimWorld.StockGenerator_Tag/<GenerateThings>c__Iterator0.MoveNext () <0x002f2>
at RimWorld.ThingSetMaker_TraderStock.Generate (RimWorld.ThingSetMakerParams,System.Collections.Generic.List`1<Verse.Thing>) <0x0035a>
at (wrapper dynamic-method) RimWorld.ThingSetMaker.Generate_Patch1 (object,RimWorld.ThingSetMakerParams) <0x00173>

What does this mean ?