Exception when deleting a modifier that others are synced to

[via5]

System.NullReferenceException: Object reference not set to an instance of an object
  at Synergy.OtherModifierSyncedModifier.ResolveModifier () [0x00000] in <filename unknown>:0 
  at Synergy.OtherModifierSyncedModifier.get_OtherModifierContainer () [0x00000] in <filename unknown>:0 
  at Synergy.OtherModifierSyncedModifier.OtherModifierRemoved (Synergy.ModifierContainer mc) [0x00000] in <filename unknown>:0 
  at Synergy.Step.DeleteModifier (Synergy.ModifierContainer mc) [0x00000] in <filename unknown>:0 
  at Synergy.ModifierUI.DeleteModifier () [0x00000] in <filename unknown>:0 
  at Synergy.ConfirmableButton.Clicked () [0x00000] in <filename unknown>:0 
  at Synergy.Button.<Clicked>m__0 () [0x00000] in <filename unknown>:0 
  at Synergy.Utilities.Handler (System.Action a) [0x00000] in <filename unknown>:0