(19:24:18) | RidableHorse.Population (ConvarControlledSpawnPopulation) has 27 objects, but max allowed is 0
(19:24:18) | - deleting 27 objects
(19:24:18) | done.
(19:24:19) | Exception while calling NextTick callback (NullReferenceException: )
  at (wrapper managed-to-native) UnityEngine.Component.get_gameObject(UnityEngine.Component)
  at UnityEngine.Component.GetComponentsInChildren[T] (System.Boolean includeInactive) [0x00001] in <ba657ba1ce6746c39d9263534c2395e1>:0 
  at Oxide.Plugins.HorseStorage+<OnEntitySpawned>c__AnonStorey0.<>m__0 () [0x00000] in <d99d1ec500d14945900b0b1450f555fc>:0 
  at Oxide.Core.OxideMod.OnFrame (System.Single delta) [0x00051] in <0fbce81d88d64454b3e7abb24df7026b>:0

This is because the horse is getting removed before Next Tick is called,  i can add a null chack for this but not quite sure why you are running the plugin if you have no horse population.

we use serverrewards for horse now so removed population and horse spawned by server can't be locked...

pushed a update for the possible NRE, thankyou for the report.