13:39 [Error] Failed to run a 0.60 timer in 'Slap v2.0.0' (NullReferenceException: )
  at (wrapper managed-to-native) UnityEngine.Component.get_transform(UnityEngine.Component)
  at BaseCombatEntity.Hurt (System.Single amount, Rust.DamageType type, BaseEntity attacker, System.Boolean useProtection) [0x0000c] in <7c8d6a6e70df4a28af996ed34cc95c98>:0
  at BaseCombatEntity.Hurt (System.Single amount) [0x00007] in <7c8d6a6e70df4a28af996ed34cc95c98>:0
  at Oxide.Game.Rust.Libraries.Player.Hurt (BasePlayer player, System.Single amount) [0x00000] in <2f5c23d6d9dc4cc9a6dac2e46556b9bf>:0
  at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Hurt (System.Single amount) [0x00000] in <2f5c23d6d9dc4cc9a6dac2e46556b9bf>:0
  at Oxide.Plugins.Slap+<SlapPlayer>c__AnonStorey0.<>m__0 () [0x00000] in <e47a85a4d9ca4bfa85f1af43570e86bd>:0
  at Oxide.Core.Libraries.Timer+TimerInstance.FireCallback () [0x00018] in <ec05e0208c9149bba43236ca58fea105>:0

My guess is you need to check if player is connected and still alive. I'm calling it from another plugin like this:

if (player.IsConnected)
{
    if (Slap != null && Slap.IsLoaded)
    {
        Slap.Call("SlapPlayer", player.IPlayer, 1000f, 1000, 1);
    }
}

The moment errors (there were several) logged player was disconnected a moment ago.

Patch (33124) has been sent. Tested, works as expected.

This is actually something they should be added in Oxide.