I see this in logfile by using : 

Failed to call hook 'CommandSelectNavigation' on plugin 'ServerRewards v2.0.4' (NullReferenceException: Object reference not set to an instance of an object)

at Oxide.Plugins.ServerRewards+<>c.<CreateCommands>b__71_0 (Oxide.Plugins.ServerRewards+Products+Command a, Oxide.Plugins.ServerRewards+Products+Command b) [0x00000] in <732665fa93454812a1035bf6a5018094>:0

at System.Collections.Generic.ArraySortHelper`1[T].SwapIfGreater (T[] keys, System.Comparison`1[T] comparer, System.Int32 a, System.Int32 b) [0x00004] in <8ce0bd04a7a04b4b9395538239d3fdd8>:0

at System.Collections.Generic.ArraySortHelper`1[T].IntroSort (T[] keys, System.Int32 lo, System.Int32 hi, System.Int32 depthLimit, System.Comparison`1[T] comparer) [0x00019] in <8ce0bd04a7a04b4b9395538239d3fdd8>:0

at System.Collections.Generic.ArraySortHelper`1[T].IntrospectiveSort (T[] keys, System.Int32 left, System.Int32 length, System.Comparison`1[T] comparer) [0x00013] in <8ce0bd04a7a04b4b9395538239d3fdd8>:0

at System.Collections.Generic.ArraySortHelper`1[T].Sort (T[] keys, System.Int32 index, System.Int32 length, System.Comparison`1[T] comparer) [0x00000] in <8ce0bd04a7a04b4b9395538239d3fdd8>:0

open your /oxide/data/ServerRewards/products.json file, scroll down to the commands section, see if any of your commands have a null Displayname property. If so, replace it with a name and reload the plugin

ex. "DisplayName": "A example display name here",

Thanks for help    !
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