Invalid argument supplied for foreach()

Warning: Invalid argument supplied for foreach() in C:\Program Files\Ampps\www\Login OOP MCV\models\m_template.php on line 59

foreach ($this->alertTypes as $alert)

<?
//Template Class - handless all templating tasks (displaying templates, alerts, errors)

class Template
{
    private $data;
    private $alertTypes;

    //Constructor
    function __construct() {}

    //Functions
    function load($url)
    {
        include($url);
    }

    function redirect($url)
    {
        header("Location: $url");
    }

    //Get/Set Data

    function setData($name, $value)
    {
        $this->data[$name] = htmlentities($value, ENT_QUOTES);
    }
    
    function getData($name)
    {
        if (isset($this->data[$name]))
        {
            return $this->data[$name];
        }
        else
        {
            return '';
        }
    }

    // Get/Set Alerts
    function setAlertTypes($types)
    {
        $this->$alertTypes = $types;
    }

    function setAlert($value, $type = null)
    {
        if ($type == ''){$type = $this->alertTypes[0];}

        $_SESSION[$type][] = $value;
    }

    function getAlerts()
    {
        $data = '';
         
        foreach ($this->alertTypes as $alert)
        {
            if (isset($_SESSION[$alert]))
            {
                foreach($_SESSION[$alert] as $value)
                {
                    $data .= '<li class"' . $alert . '">' . $value . '</li>';
                }
                unset($_SESSION[$alert]);
            }
        }
        return $data;
    }
}

// Check if $myList is indeed an array or an object.
if (is_array($alertTypes) || is_object($alertTypes))
{
  // If yes, then foreach() will iterate over it.
  foreach($this->alertTypes as $alert){
      //Do something.
  }
}

<h1><?php echo $this->getData('name');?></h1>
    <?php 
        $alerts = $this->getAlerts();
        if ($alerts != '') {echo '<ul class="alerts">' . $alerts . '';}
    ?>
    <div id="content">
        <p>Test</p>

        <a href="members.php">Link</a>
    </div>