Notice: Undefined index: Error in contact form

Hi there,

I’ve been following this tutorial to create my own contact form – template, and it works great, it sends the email. But I do get a weird error in all the fields when loading the page.

This is the error:
Notice: Undefined index: message_email in *domain*/wp-content/themes/dobocor/template-contact-2.php on line 100

So, it has something to do with the PHP of my input fields, but I have no idea what it is. Here is my PHP template:

<?php
/**
 * Template Name: Contact Form 2
 *
 */
?>

<?php

  //response generation function
  $response = "";

  //function to generate response
  function my_contact_form_generate_response($type, $message){

    global $response;

    if($type == "success") $response = "<div class='success'>{$message}</div>";
    else $response = "<div class='error'>{$message}</div>";

  }
	//response messages
	$not_human       = "Human verification incorrect.";
	$missing_content = "Please supply all information.";
	$email_invalid   = "Email Address Invalid.";
	$message_unsent  = "Message was not sent. Try Again.";
	$message_sent    = "Thanks! Your message has been sent.";

	//user posted variables
	$name = $_POST['message_name'];
	$email = $_POST['message_email'];
	$message = $_POST['message_text'];
	$human = $_POST['message_human'];

	//php mailer variables
	$to = get_option('admin_email');
	$subject = "Someone sent a message from ".get_bloginfo('name');
	$headers = __('From: ','dobocor') .$name. '<no-reply@'.$_SERVER['SERVER_NAME'].'>' . "\r\n" .
	  'Reply-To: ' . $email . "\r\n";

	if(!$human == 0){
	  if($human != 2) my_contact_form_generate_response("error", $not_human); //not human!
	  else {

	    //validate email
		if(!filter_var($email, FILTER_VALIDATE_EMAIL))
		  my_contact_form_generate_response("error", $email_invalid);
		else //email is valid
		//validate presence of name and message
		if(empty($name) || empty($message)){
		  my_contact_form_generate_response("error", $missing_content);
		}
		else //ready to go!
		{
	    $sent = wp_mail($to, $subject, strip_tags($message), $headers);
		if($sent) my_contact_form_generate_response("success", $message_sent); //message sent!
		else my_contact_form_generate_response("error", $message_unsent); //message wasn't sent
		}
	  }
	}
	else if ($_POST['submitted']) my_contact_form_generate_response("error", $missing_content);

?>

<?php get_header(); ?>

<?php
if (rwmb_meta('wpos_service_button') != '') {
	$button =  rwmb_meta('wpos_service_button');
} ?>

<div class="bg-gray home-section">
<div class="container">
	<div class="row">

	<div id="primary" class="col-md-12 col-lg-12">
		<main id="main" class="site-main" role="main">
			<div class="row">

				<div class="container marginbot-50">
					<div class="row">
						<div class="col-lg-8 col-lg-offset-2">
							<div>
							<div class="section-heading text-center">
							<h2 class="h-bold">
								<?php the_title(); ?>
								<?php echo $_SERVER['SERVER_NAME']; ?>
							</h2>
							<div class="divider-header"></div>
							</div>
							</div>
						</div>
					</div>
				</div>

				<div id="respond">
				  <?php echo $response; ?>
				  <form action="<?php the_permalink(); ?>" method="post">
				    <p><label for="name">Name: <span>*</span> <br><input type="text" name="message_name" value="<?php echo esc_attr($_POST['message_name']); ?>"></label></p>
				    <p><label for="message_email">Email: <span>*</span> <br><input type="text" name="message_email" value="<?php echo esc_attr($_POST['message_email']); ?>"></label></p>
				    <p><label for="message_text">Message: <span>*</span> <br><textarea type="text" name="message_text"><?php echo esc_textarea($_POST['message_text']); ?></textarea></label></p>
				    <p><label for="message_human">Human Verification: <span>*</span> <br><input type="text" style="width: 60px;" name="message_human"> + 3 = 5</label></p>
				    <input type="hidden" name="submitted" value="1">
				    <p><input type="submit"></p>
				  </form>
				</div>

		</main><!-- #main -->
	</div><!-- #primary -->

	</div><!-- .row -->
</div><!-- .container -->
</div><!-- .bg-gray -->
<?php get_footer(); ?>

can anyone help me?