php - $_GET undefined index radio button

im doing ORDER BY list by using radio button and then submit.

im stuck in this code, it works fine when i submit but there's an error message undefined index whenever i check and submit the Pending radio button. the $limit variable is just the limit of list per page.

PHP

$limit = 'LIMIT ' .($pagenum - 1) * $page_rows .',' .$page_rows;
$sql = "SELECT * FROM v_tickets_information";
if (!isset($_GET['filter'])) {

if ($_GET['filter'] == 'old') {
    $sql .= " ORDER BY Date_Time ASC $limit";
} elseif ($_GET['filter'] == 'latest') {
    $sql .= " ORDER BY Date_Time DESC $limit";
} elseif ($_GET['stats'] == 'pending') {
    $sql .= " WHERE TicketStatus = 'Pending' ORDER BY Date_Time DESC $limit";
} 
}
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HTML

<form>
<input type="radio" name="filter" value="latest"> Latest to old<br>
<input type="radio" name="filter" value="old"> Old to latest<br>


<hr>  

<input type="radio" name="stats" value="new"> New<br>
<input type="radio" name="stats" value="pending"> Pending<br>
<input type="radio" name="stats" value="open"> Open<br>
<input type="radio" name="stats" value="closed"> Closed


<div >
<label for="dept"><h2>Department</h2></label>
<select id="dept" name="dept">
<option>All</option>
<option>Accounting</option>
<option>Admin Office</option>
<option>Customer Service</option>
<option>SLI</option>
<option>HRD</option>
<option>Engineering Office</option>
<option>Production</option>
<option>Accounting</option>
<option>IMPEX</option>
<option>MIS</option>

</select><br>
<div >
 <input type="submit" name="Submit" value="Sort"   />
</div>
</div>
 </form>
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btw i didnt create a code php for department cause im getting early error i have to fix this first. help me please fix this code. Heres the screenshot. screenshot image