我收到未捕获的错误:在PHP语言中提交表单时出现null错误时调用成员函数isSMTP()

require_once 'path/to/PHPMailer/PHPMailer.php';

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;

// 创建PHPMailer对象
$mail = new PHPMailer(true);

try {
    // 设置SMTP参数
    $mail->isSMTP();
    $mail->Host = 'smtp.example.com';
    $mail->SMTPAuth = true;
    $mail->Username = 'your_username';
    $mail->Password = 'your_password';
    $mail->SMTPSecure = 'tls';
    $mail->Port = 587;

    // 其他邮件设置
    $mail->setFrom('from@example.com', 'Your Name');
    $mail->addAddress('recipient@example.com', 'Recipient Name');
    $mail->Subject = 'Test Email';
    $mail->Body = 'This is a test email.';

    // 发送邮件
    $mail->send();
    echo 'Email sent successfully.';
} catch (Exception $e) {
    echo 'Email could not be sent. Error: ' . $mail->ErrorInfo;
}