Oddly enough, I won't get the error when they start a marriage ceremony. I can fix it by recalling them every time a party begins, but it's a bit annoying to have to do that.

Error Log for that specific issue:

Exception in Verse.AI.ThinkNode_Priority TryIssueJobPackage: System.NullReferenceException: Object reference not set to an instance of an object
at RimWorld.GatheringsUtility.ShouldGuestKeepAttendingGathering (Verse.Pawn p) [0x00000] in <filename unknown>:0
at RimWorld.PartyUtility.ShouldPawnKeepPartying (Verse.Pawn p) [0x00000] in <filename unknown>:0
at RimWorld.LordJob_Joinable_Party.VoluntaryJoinPriorityFor (Verse.Pawn p) [0x00000] in <filename unknown>:0
at RimWorld.ThinkNode_JoinVoluntarilyJoinableLord.JoinVoluntarilyJoinableLord (Verse.Pawn pawn) [0x00000] in <filename unknown>:0
at RimWorld.ThinkNode_JoinVoluntarilyJoinableLord.TryIssueJobPackage (Verse.Pawn pawn, JobIssueParams jobParams) [0x00000] in <filename unknown>:0
at Verse.AI.ThinkNode_Priority.TryIssueJobPackage (Verse.Pawn pawn, JobIssueParams jobParams) [0x00000] in <filename unknown>:0
Verse.Log:Error(String)
Verse.AI.ThinkNode_Priority:TryIssueJobPackage(Pawn, JobIssueParams)
Verse.AI.Pawn_JobTracker:DetermineNextJob(ThinkTreeDef&)
Verse.AI.Pawn_JobTracker:TryFindAndStartJob()
Verse.AI.Pawn_JobTracker:EndCurrentJob(JobCondition, Boolean)
Verse.AI.Pawn_JobTracker:JobTrackerTick()
Verse.Pawn:Tick()
AIRobot.X2_AIRobot:Tick()
Verse.TickList:Tick()
Verse.TickManager:DoSingleTick()
Verse.TickManager:TickManagerUpdate()
Verse.Game:UpdatePlay()
Verse.Root_Play:Update()