Q:

Typo3 8.7: Powermail 6.1: the default controller for extension "Powermail" and plugin "Pi1" can not be determined

I'm currently working on migrating my Typo3 6.x to 8.7.
I know there are plenty of similar questions asked here, but I have tryed everything, every solution proposed. 
To my problem:
When I visit a page that has a powermail form, I'm getting this Error:
The default controller for extension "Powermail" and plugin "Pi1" can not be 
determined. Please check for 
TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin() in your 
ext_localconf.php.

Since Powermail doesn't have any errors obviously it has to do something with my configurations.
I'm using a different extension to add a cronjob (entrie in DB), whenever a form is being processed in the FormController of Powermail.     
ext_localconf.php of my extension:
<?php
defined('TYPO3_MODE') || die('Access denied.');

call_user_func(
    function ($extKey) {
        \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTypoScriptSetup('<INCLUDE_TYPOSCRIPT: source="FILE:EXT:extKeyHere/Configuration/TypoScript/setup.ts">');
        \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTypoScriptConstants('<INCLUDE_TYPOSCRIPT: source="FILE:EXT:extKeyHere/Configuration/TypoScript/constants.ts">');

        \TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
            'TYPO3.' . $extKey,
            'Integration',
            array(
                'Integration' => 'integration',
            ),
            // non-cacheable actions
            array(
                'Integration' => 'integration',
            )
        );
    },
    $_EXTKEY
);

$signalSlotDispatcher = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\CMS\Extbase\SignalSlot\Dispatcher');
$signalSlotDispatcher->connect(
    'In2code\Powermail\Controller\FormController', // namespace of Class: In2code\Powermail\Controller
    'createActionAfterMailDbSaved',
    'Typo3\extName\Controller\IntegrationController', //namespace of Class: Typo3\extName\Controller
    'integration',
    FALSE);
?>

I can't figure out what the problem is. I have deleted the Powermail extension, reactivated it, cleared all cashes (also by using install tool: Clear all cache) etc. without success.
Thanks in advance.

A:

I found the solution myself. The table in my database in which the forms of powermail are stored was empty. I had to add forms to this table tx_powermail_domain_model_form. I have had my form records inside the table tx_powermail_domain_model_formS, therefore no forms were found inside the Plugin configuration of the specific template in the TYPO3 backend.
(Page -> Select Template with Powermailform -> Plugin -> Main Settings -> Choose a Powermailform (was empty)).
Maybe this is helpful for others!

