My flexform configuration doesn't show up - typo3

Question

I tried to to make it possible to configure the FE plugins of my own extension. The flexform configurion I tried doesn't show up in the backend.
Why it doesn't work and what do I need to change?
I'm working with Typo3 8.7.3.
Here are the relevant code parts:
ext_tables.php
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
'Fmogge.'.$extKey,
'Nextcourse',
'Nächste Kurse'
);
$extensionName = \TYPO3\CMS\Core\Utility\GeneralUtility::underscoredToUpperCamelCase($_EXTKEY);
$frontendpluginName = 'Nextcourse';
$pluginSignature = strtolower($extensionName).'_'.strtolower($frontendpluginName);
$TCA['tt_content']['types']['list']['subtypes_addlist'][$pluginSignature] = 'pi_flexform';
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue($pluginSignature, 'FILE:EXT:'.$_EXTKEY.'/Configuration/FlexForms/Nextcourse.xml');
Configuration/FlexForms/Nextcourse.xml
<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<T3DataStructure>
<sheets>
<sDEF>
<ROOT>
<TCEforms>
<sheetTitle>Optionen</sheetTitle>
</TCEforms>
<type>array</type>
<el>
<settings.ort>
<TCEforms>
<label>Ort wählen</label>
<config>
<type>select</type>
<items type="array">
<numIndex index="0" type="array">
<numIndex index="0"></numIndex>
<numIndex index="1"></numIndex>
</numIndex>
</items>
<foreign_table>tx_coursemanager_domain_model_ort</foreign_table>
<foreign_table_where>
AND elterndatensatz = 0
</foreign_table_where>
</config>
</TCEforms>
</settings.ort>
</el>
</ROOT>
</sDEF>
</sheets>

Answer 1

You can Use GLOBAL variables like below. it's work for me.
Just replace below line in your ext_tables.php file
$GLOBALS['TCA']['tt_content']['types']['list']['subtypes_addlist'][$pluginSignature] = 'pi_flexform';

Answer 2

In your ext_tables.php change syntax of $extKey to $_EXTKEY which will solve your issue, like:
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
'Fmogge.'.$_EXTKEY,
'Nextcourse',
'Nächste Kurse'
);
$extensionName = \TYPO3\CMS\Core\Utility\GeneralUtility::underscoredToUpperCamelCase($_EXTKEY);
$frontendpluginName = 'Nextcourse';
$pluginSignature = strtolower($extensionName).'_'.strtolower($frontendpluginName);
$TCA['tt_content']['types']['list']['subtypes_addlist'][$pluginSignature] = 'pi_flexform';
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue($pluginSignature, 'FILE:EXT:'.$_EXTKEY.'/Configuration/FlexForms/Nextcourse.xml');
Hint: Avoid using $_EXTKEY in the future, use extension key as string instead. It has been changed in core and various 3rd party extensions. It helps, when you move such configs to folder Configuration/TCA/.. where $_EXTKEY is not available.
Your flexform file misses closing tag for T3DataStructure like:
<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<T3DataStructure>
<sheets>
<sDEF>
<ROOT>
<TCEforms>
<sheetTitle>Optionen</sheetTitle>
</TCEforms>
<type>array</type>
<el>
<settings.ort>
<TCEforms>
<label>Ort wählen</label>
<config>
<type>select</type>
<items type="array">
<numIndex index="0" type="array">
<numIndex index="0"></numIndex>
<numIndex index="1"></numIndex>
</numIndex>
</items>
<foreign_table>tx_coursemanager_domain_model_ort</foreign_table>
<foreign_table_where>
AND elterndatensatz = 0
</foreign_table_where>
</config>
</TCEforms>
</settings.ort>
</el>
</ROOT>
</sDEF>
</sheets>
<T3DataStructure>