Failed to call hook 'cmdBuyItem' on plugin 'ServerRewards v0.4.71' (NullReferenceException: Object reference not set to an instance of an object)
at BasePlayer.GiveItem (Item item, BaseEntity+GiveItemReason reason) [0x0002b] in <5ba1d4536658475998daa468484da8f2>:0
at Oxide.Plugins.ServerRewards.GiveItem (BasePlayer player, System.String itemkey) [0x000e5] in <341d86d447d94f0bbaaac9b9271f90cb>:0
at Oxide.Plugins.ServerRewards.cmdBuyItem (ConsoleSystem+Arg arg) [0x00113] in <341d86d447d94f0bbaaac9b9271f90cb>:0
at Oxide.Plugins.ServerRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00e58] in <341d86d447d94f0bbaaac9b9271f90cb>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <efb9a8684e344ecfa065c7b416e6d2fd>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <b0a93b504f034a0db8b1c3d2503c0987>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <b0a93b504f034a0db8b1c3d2503c0987>:0

Any idea ?

hmm I also ocassionaly see this in console:

iled to call hook 'cmdBuyItem' on plugin 'ServerRewards v0.4.71' (NullReferenceException: Object reference not set to an instance of an object)
  at BasePlayer.GiveItem (Item item, BaseEntity+GiveItemReason reason) [0x0002b] in <ea55bc664c5848688ffe06707443ae1c>:0 
  at Oxide.Plugins.ServerRewards.GiveItem (BasePlayer player, System.String itemkey) [0x000e5] in <ce3fd74d17464a13b91d26eaf15b5ac1>:0 
  at Oxide.Plugins.ServerRewards.cmdBuyItem (ConsoleSystem+Arg arg) [0x00113] in <ce3fd74d17464a13b91d26eaf15b5ac1>:0 
  at Oxide.Plugins.ServerRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00e58] in <ce3fd74d17464a13b91d26eaf15b5ac1>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <8a646463979e4a1d9c0e2f1298e9483f>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <1fe782f4b27a43ae85d29ebb2d56c007>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <1fe782f4b27a43ae85d29ebb2d56c007>:0 ​

I have been getting this all day today

Merged post

Spoke with @k1lly0u on this issue guys, its a possibly corrupter data file, in my case the cooldown_data.json was genrating with a null causing this issue.