Call to a member function getTag() on null

Error:
Call to a member function getTag() on null

  at /www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/AssetItem/LangActiveAssetItem.php:48
  at Joomla\CMS\WebAsset\AssetItem\LangActiveAssetItem->__construct('template.active.language', '', array('description' => 'An asset to allow language specific css, eg \'language/[lang-CODE]/[lang-CODE].css\', to use it as a dependency to the active template', 'type' => 'style', 'class' => 'LangActiveAssetItem', 'dependencies' => array('template.active'), 'assetSource' => array('registryFile' => 'templates/cassiopeia/joomla.asset.json')), array(), array())
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/WebAssetRegistry.php:247)
  at Joomla\CMS\WebAsset\WebAssetRegistry->createAsset('template.active.language', '', array('description' => 'An asset to allow language specific css, eg \'language/[lang-CODE]/[lang-CODE].css\', to use it as a dependency to the active template', 'type' => 'style', 'class' => 'LangActiveAssetItem', 'dependencies' => array('template.active'), 'assetSource' => array('registryFile' => 'templates/cassiopeia/joomla.asset.json')))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/WebAssetRegistry.php:419)
  at Joomla\CMS\WebAsset\WebAssetRegistry->parseRegistryFile('templates/cassiopeia/joomla.asset.json')
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/WebAssetRegistry.php:351)
  at Joomla\CMS\WebAsset\WebAssetRegistry->parseRegistryFiles()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/WebAssetRegistry.php:159)
  at Joomla\CMS\WebAsset\WebAssetRegistry->add('style', object(WebAssetItem))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/WebAssetManager.php:508)
  at Joomla\CMS\WebAsset\WebAssetManager->registerAsset('style', 'theme.colors_standard', 'media/templates/site/cassiopeia/css/global/colors_standard.css')
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/WebAsset/WebAssetManager.php:234)
  at Joomla\CMS\WebAsset\WebAssetManager->__call('registerandusestyle', array('theme.colors_standard', 'media/templates/site/cassiopeia/css/global/colors_standard.css'))
     (/www/htdocs/w019c449/praxis-ringwald.de/templates/cassiopeia/error.php:40)
  at require('/www/htdocs/w019c449/praxis-ringwald.de/templates/cassiopeia/error.php')
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Document/HtmlDocument.php:721)
  at Joomla\CMS\Document\HtmlDocument->_loadTemplate('/www/htdocs/w019c449/praxis-ringwald.de/templates/cassiopeia', 'error.php')
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Document/HtmlDocument.php:776)
  at Joomla\CMS\Document\HtmlDocument->_fetchTemplate(array('template' => 'cassiopeia', 'directory' => '/www/htdocs/w019c449/praxis-ringwald.de/templates', 'debug' => '0', 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Document/HtmlDocument.php:612)
  at Joomla\CMS\Document\HtmlDocument->parse(array('template' => 'cassiopeia', 'directory' => '/www/htdocs/w019c449/praxis-ringwald.de/templates', 'debug' => '0', 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Document/HtmlDocument.php:630)
  at Joomla\CMS\Document\HtmlDocument->render(false, array('template' => 'cassiopeia', 'directory' => '/www/htdocs/w019c449/praxis-ringwald.de/templates', 'debug' => '0', 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Document/ErrorDocument.php:139)
  at Joomla\CMS\Document\ErrorDocument->render(false, array('template' => 'cassiopeia', 'directory' => '/www/htdocs/w019c449/praxis-ringwald.de/templates', 'debug' => '0', 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Error/Renderer/HtmlRenderer.php:70)
  at Joomla\CMS\Error\Renderer\HtmlRenderer->render(object(RuntimeException))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Exception/ExceptionHandler.php:126)
  at Joomla\CMS\Exception\ExceptionHandler::render(object(RuntimeException))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Exception/ExceptionHandler.php:72)
  at Joomla\CMS\Exception\ExceptionHandler::handleException(object(RuntimeException))
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Application/CMSApplication.php:322)
  at Joomla\CMS\Application\CMSApplication->execute()
     (/www/htdocs/w019c449/praxis-ringwald.de/includes/app.php:61)
  at require_once('/www/htdocs/w019c449/praxis-ringwald.de/includes/app.php')
     (/www/htdocs/w019c449/praxis-ringwald.de/index.php:32)                

RuntimeException:
Failed to start the session because headers have already been sent by "/www/htdocs/w019c449/praxis-ringwald.de/libraries/vendor/joomla/database/src/Mysql/MysqlDriver.php" at line 121.

  at /www/htdocs/w019c449/praxis-ringwald.de/libraries/vendor/joomla/session/src/Storage/NativeStorage.php:473
  at Joomla\Session\Storage\NativeStorage->start()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Session/Storage/JoomlaStorage.php:295)
  at Joomla\CMS\Session\Storage\JoomlaStorage->start()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/vendor/joomla/session/src/Session.php:406)
  at Joomla\Session\Session->start()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/vendor/joomla/session/src/Session.php:333)
  at Joomla\Session\Session->has('user')
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Session/Session.php:194)
  at Joomla\CMS\Session\Session->get('user')
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Factory.php:372)
  at Joomla\CMS\Factory::getUser()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Application/SiteApplication.php:562)
  at Joomla\CMS\Application\SiteApplication->initialiseApp()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Application/SiteApplication.php:226)
  at Joomla\CMS\Application\SiteApplication->doExecute()
     (/www/htdocs/w019c449/praxis-ringwald.de/libraries/src/Application/CMSApplication.php:293)
  at Joomla\CMS\Application\CMSApplication->execute()
     (/www/htdocs/w019c449/praxis-ringwald.de/includes/app.php:61)
  at require_once('/www/htdocs/w019c449/praxis-ringwald.de/includes/app.php')
     (/www/htdocs/w019c449/praxis-ringwald.de/index.php:32)