未定义的索引：动作[重复]

The highlighted line is line 24

Notice: Undefined index: action in C:\xampp\htdocs\authors.php on line 24

  **if($_GET['action'] == "list") {**
        $current = "members";
        $list = "members";
    }
    else if($_GET["list"] == "authors") $current = "authors";
    else $current = "members";

</div>

Check first if $_GET['action'] has been set or not

if(isset($_GET['action'])) {

    if($_GET['action'] == "list") {
        $current = "members";
        $list = "members";
    } else if($_GET["list"] == "authors") {
        $current = "authors";
    } else {
        $current = "members";
    }

}