Could be your mod, or Giddy Up (it's also reported there)

Exception in JobDriver fixed tick for pawn AA_Groundrunner1284093 driver=JobDriver_Mounted (toilIndex=1) driver.job=(Mounted (Job_3282188) A = Thing_Human1284035 Giver = ThinkNode_QueuedJob [workGiverDef: null])
System.ArgumentNullException: Value cannot be null.
Parameter name: key
[Ref 43BC2F91]
[0x00006] in <51fded79cd284d4d911c5949aff4cb21>:0
at System.Collections.Generic.Dictionary`2[TKey,TValue].FindEntry (TKey key) [0x00008] in <51fded79cd284d4d911c5949aff4cb21>:0
at System.Collections.Generic.Dictionary`2[TKey,TValue].TryGetValue (TKey key, TValue& value) [0x00000] in <51fded79cd284d4d911c5949aff4cb21>:0
at GiddyUp.Jobs.JobDriver_Mounted.RiderShouldDismount (GiddyUp.ExtendedPawnData riderData) [0x002b3] in C:\Program Files (x86)\Steam\steamapps\common\RimWorld\Source\Giddy-Up\Source\Core\Jobs\JobDriver_Mounted.cs:211
at GiddyUp.Jobs.JobDriver_Mounted.<DelegateMovement>b__19_0 () [0x0001a] in C:\Program Files (x86)\Steam\steamapps\common\RimWorld\Source\Giddy-Up\Source\Core\Jobs\JobDriver_Mounted.cs:109
at Verse.AI.JobDriver.DriverTick () [0x001a3] in <78bd3c897ba748f7a6d21cc5498c054e>:0
- POSTFIX safetemperature.1trickPwnyta: Void SafeTemperature.Patch_JobDriver_DriverTick:Postfix(Pawn ___pawn)
UnityEngine.StackTraceUtility:ExtractStackTrace ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Log.Error_Patch1 (string)
Verse.AI.JobUtility:TryStartErrorRecoverJob (Verse.Pawn,string,System.Exception,Verse.AI.JobDriver)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.AI.JobDriver.DriverTick_Patch1 (Verse.AI.JobDriver)
Verse.AI.Pawn_JobTracker:JobTrackerTick ()
Verse.Pawn:Tick ()
Verse.Thing:DoTick ()
Verse.TickList:Tick ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.TickManager.DoSingleTick_Patch6 (Verse.TickManager)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.TickManager.TickManagerUpdate_Patch0 (Verse.TickManager)
Verse.Game:UpdatePlay ()
Verse.Root_Play:Update ()