So Im hoping I can get some help here with a mix of CustomeHelicopterTiers and ServerRewards.
I set my heli tiers to Easy, Medium, Hard and Expert
I used the following in my ServerRewards config:
    "Tier 1 Heli": {
      "description": "Summons tier 1 attack helicopter",
      "iconName": "https://imgur.com/rHvNSSx",
      "commands": [
        "heli.call Easy $player.id",
        "say Tier 1 heli called by $player.name, to their location. Do <color=#ff0000>NOT</color> interfere with it."
      ],
      "displayName": "Tier 1 Heli",
      "cost": 1000,
      "cooldown": 3600
    },
but when Im in game and try to summon the heli I get the following error:
(20:44:21) | Failed to call hook 'cmdBuyCommand' on plugin 'ServerRewards v0.4.73' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.ServerRewards.cmdBuyCommand (ConsoleSystem+Arg arg) [0x00057] in <f1baa28124be4a599de75334939183e7>:0
  at Oxide.Plugins.ServerRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00e2a] in <f1baa28124be4a599de75334939183e7>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <31122a27a2414cd799150f8677cf39d4>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0