The default controller for extension and plugin can not be determined - typo3

Question

Good afternoon, dear friends! All, I give up. Tried well, all that was already possible. TYPO3 7.6.16
ext_tables.php:
<?php
if (!defined('TYPO3_MODE')) die ('Access denied.');
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
    'MyVendor.' . $_EXTKEY,
    'Pi1',
    'The inventory list'
);
ext_localconf.php:
<?php
if (!defined('TYPO3_MODE')) die ('Access denied.');
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
    'MyVendor.' . $_EXTKEY,
    'Pi1',
    Array ('Comment' => 'list'),
    Array ('Comment' => 'list')
);
And constantly the same mistake
The default controller for extension "Fecomments" and plugin "Pi1" can not be determined
I read topics with same error but nothing help me.
I already climbed into the kernel, found out that $configuration ['controllerConfiguration'] is an empty array, I do not know why data does not arrive there. Comrades, help me out, I do not know what to do, honestly! )

Answer 1

At first, use the correct syntax for the two files. Examples:
ext_tables.php:
<?php
defined('TYPO3_MODE') || die('Access denied.');
call_user_func(
function($extKey)
{
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
'VENDOR.Extensionkey',
'Pi1',
'Extension Display Name'
);
},
$_EXTKEY
);
ext_localconf.php:
<?php
defined('TYPO3_MODE') || die('Access denied.');
call_user_func(
function($extKey)
{
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
'VENDOR.' . $extKey,
'Pi1',
[
'First' => 'action1, action2'
],
// non-cacheable actions
[
'First' => ''
]
);
},
$_EXTKEY
);
The make sure the namespace and class name are fine:
typo3conf/ext/extensionkey/Classes/Controller/FirstController.php:
/***
*
* This file is part of the "extensionkey" Extension for TYPO3 CMS.
*
* For the full copyright and license information, please read the
* LICENSE.txt file that was distributed with this source code.
*
* (c) 2017
*
***/
use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;
/**
* FilecollectorController
*/
class FirstController extends ActionController
{
/**
* action1
*
* #return void
*/
public function action1Action()
{
}
/**
* action1
*
* #return void
*/
public function action2Action()
{
}
}
Clear all caches. Sometimes it will help to go into ExtensionManager and disable/enable the whole extension. In case of changed classnames or changes in the tables/localconf files, this will flush ALL caches.

Answer 2

The solution provided by Mikael is not available at the moment.
Short version of the external website, that is currently offline:
Try to delete and recreate the content element on your page to delete old flexform values