Error before a $_Post is sent to the mysql query [duplicate]

I have the following code for a multiple dropdown menu, which takes the data from mysql database, then depending on the choice, displays a another dropdown menu.

I have done this with php, javascript and MySQL.

The problem I come across is that the first time I load the page I get the error that the array, that comes from $_Post, is not created. Such array enables the mysql query and following chart to appear.

The error is:
“Warning: Undefined array key “titulos” … “

After I do one submission with the dropdown menu, everything works fine.

How can I create a default result for this variable?

Dropdown menu and Chart Query

<form style="text-align:center;margin-top:2%;" name="myform" action="" method="post"><select style="font-size:16px;" name="magazines"   onclick="getSeries(this.value)">
    <option value="">Choose a Magazine</option>
    <?php
        $magazines = getMagazines();
        foreach($magazines as $magazine){
               <option value="<?php echo $magazine['magazine_code'] ?>">
              <?php echo $magazine['Magazine_name']?></option>
    <?php
    }
    ?>
    </select>
    <select style="font-size:16px;" name="titulos">
    <option value="">Choose a Series</option>
    </select>
    <input style="font-size:16px;" value="Send"  type="submit">


     $series_tit =$_POST['titulos'];
     $query = "SELECT * FROM chartdata WHERE Serie_name = '$series_tit'";

PHP functions

function getMagazines(){
    $conn = databaseConnection();
    $res = $conn->query("SELECT * FROM categorias");
    while($row = $res->fetch_assoc()){
        $data[] = $row;
    }
    return $data;


}

if(isset($_GET["magazine_code"])){
echo getSeries($_GET["magazine_code"]);
}

function getSeries($magazine_code){
$conn = databaseConnection();
if(!$conn){
return false;
}
$magazine_code = htmlspecialchars($magazine_code);
$res = $conn->query("SELECT * FROM chartdata WHERE magazine_code ='$magazine_code'");
while($row = $res->fetch_assoc()){
$data[] = $row;
}
return json_encode($data);

}

And Javascript

function getSeries(magazineCode){
    let seriesDropDown = document.forms["myform"].titulos;

    if (magazineCode.trim() === ""){
        seriesDropDown.disabled = true;
        seriesDropDown.selectedIndex = 0;
        return false;
    }

    fetch(`functions_sales.php?magazine_code=${magazineCode}`)
    .then (response => response.json())
    .then(function(series){
        let out = "";
        out += `<option value="">Choose a Series</option>`;
        for (let sery of series){
            out += `<option value="${sery['Serie_name']}">${sery['Serie_name']}</option>`;
        }
        seriesDropDown.innerHTML = out;
        seriesDropDown.disabled = false;


    });

}

I’ve tried making a selected option into the form but the response continue being the same.