Undefined index: name_of_your_nonce_field during first basic example with - wordpress

Question

while my first steps with wp nonce field i tried the "Basic Examples" from https://developer.wordpress.org/reference/functions/wp_nonce_field/
it says there: "simplest implementation which omits all arguments"
at the bottom of my htdocs/wp-content/plugins/abcd-plugin/abcd-plugin.php
i wrote:
function hi_in_wp_head() {
?>
<form name="f1">
<input name="i1" value="hi_in_wp_head">
<input type="submit" name="s1">
<?php wp_nonce_field('name_of_your_action', 'name_of_your_nonce_field'); ?>
</form>
<?php
if(wp_verify_nonce($_REQUEST['name_of_your_nonce_field'], 'name_of_your_action')){
// Nonce is matched and valid. do whatever you want now.
} else {
// Invalid nonce. you can throw an error here.
die("ups 19-02-28_17-09");
}
}
function hi_in_footer() {
echo '<h1>hi_in_footer</h1>';
}
complete source:
https://gist.github.com/f9f0a853f0a71c5a2055b88802a1010c
this looks like this in the web browser:
<meta name="generator" content="WordPress 5.0.3" />
<form name="f1">
<input name="i1" value="hi_in_wp_head">
<input type="submit" name="s1">
<input type="hidden" id="name_of_your_nonce_field" name="name_of_your_nonce_field" value="5a82357118" /><input type="hidden" name="_wp_http_referer" value="/wordpress/alecaddd-plugin.php" /> </form>
<br />
<b>Notice</b>: Undefined index: name_of_your_nonce_field in <b>G:\Bitnami\wordpress-5.0.3-2\apps\wordpress\htdocs\wp-content\plugins\alecaddd-plugin\alecaddd-plugin.php</b> on line <b>89</b><br />
ups 19-02-28_17-09
Undefined index: name_of_your_nonce_field during first basic example with
I do not know where the error comes from. what i could do?

Answer 1

As the error message states, $_REQUEST['name_of_your_nonce_field'] isn't set. You need to make sure it's set before using it:
function hi_in_wp_head() {
?>
<form name="f1">
<input name="i1" value="hi_in_wp_head">
<input type="submit" name="s1">
<?php wp_nonce_field('name_of_your_action', 'name_of_your_nonce_field'); ?>
</form>
<?php
if(isset($_REQUEST['name_of_your_nonce_field']) {
if(wp_verify_nonce($_REQUEST['name_of_your_nonce_field'], 'name_of_your_action')){
// Nonce is matched and valid. do whatever you want now.
} else {
// Invalid nonce. you can throw an error here.
die("ups 19-02-28_17-09");
}
}
}
$_REQUEST['name_of_your_nonce_field'] will be set after your form gets submitted. That's why you need the extra check.