Message: Interface\AddOns\GreenWall\Utility.lua:272: Usage: GetItemInfo(itemID|"name"|"itemlink")
Time: 01/01/18 10:56:16
Count: 2
Stack: Interface\AddOns\GreenWall\Utility.lua:272: Usage: GetItemInfo(itemID|"name"|"itemlink")
[C]: ?
[C]: in function GetItemInfo' Interface\AddOns\GreenWall\Utility.lua:272: in function IsLegendary'
Interface\AddOns\GreenWall\GreenWall.lua:264: in function `GreenWall_OnEvent'
[string ":OnEvent"]:1: in function <[string ":OnEvent"]:1>

Locals:

Do you have the full stack trace?

That's the full error message.
It just needs some more nil checks.
caged battlepets return Hbattlepet:, not item: , so item passed to IsLegendary is nul.