Have you tried printing the event sent to AST_Event_LUAWarning just in case you have somehow managed to AST_RegisterEvent() another event with no payload to that function association?
From the /API documents:
Code:
ScriptWarnings->Events
LuaWarning
LiteralName = LUA_WARNING
PAYLOAD:
warnType, Type = number, Nilable = false
warningText, Type = string, Nilable = false