In expression, add "ef_variadic_parameters", see official array plugin -
AddNumberParam("X", "The X index (0-based) of the array value to get.", "0");
AddExpression(0, ef_return_any | ef_variadic_parameters, "Get value at", "Array", "At", "Get value from the array. Add second or third parameters to specify Y and Z indices.");
In action or condition, if the type of parameter added on the last is not a "object type", it could not be filled automatically. i.e, editor could not find a default "object type" in this case.