Steps to reproduce the issue

install joomla 5.2 beta
activade compatibility plugin
install flexicontent

Expected result

install joomla 5.1 beta
install flexicontent
All works

Actual result

Call stack
--
# | Function | Location
1 | () | JROOT\libraries\src\HTML\HTMLHelper.php:165
2 | Joomla\CMS\HTML\HTMLHelper::_() | JROOT\administrator\components\com_flexicontent\views\items\tmpl\default.php:1111
3 | include() | JROOT\libraries\src\MVC\View\HtmlView.php:415
4 | Joomla\CMS\MVC\View\HtmlView->loadTemplate() | JROOT\libraries\src\MVC\View\HtmlView.php:203
5 | Joomla\CMS\MVC\View\HtmlView->display() | JROOT\administrator\components\com_flexicontent\views\items\view.html.php:1157
6 | FlexicontentViewItems->display() | JROOT\libraries\src\MVC\Controller\BaseController.php:697
7 | Joomla\CMS\MVC\Controller\BaseController->display() | JROOT\administrator\components\com_flexicontent\controller.php:286
8 | FlexicontentController->display() | JROOT\libraries\src\MVC\Controller\BaseController.php:730
9 | Joomla\CMS\MVC\Controller\BaseController->execute() | JROOT\administrator\components\com_flexicontent\admin.flexicontent.php:264
10 | require_once() | JROOT\administrator\components\com_flexicontent\flexicontent.php:22
11 | require_once() | JROOT\libraries\src\Dispatcher\LegacyComponentDispatcher.php:71
12 | Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() | JROOT\libraries\src\Dispatcher\LegacyComponentDispatcher.php:73
13 | Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() | JROOT\libraries\src\Component\ComponentHelper.php:361
14 | Joomla\CMS\Component\ComponentHelper::renderComponent() | JROOT\libraries\src\Application\AdministratorApplication.php:150
15 | Joomla\CMS\Application\AdministratorApplication->dispatch() | JROOT\libraries\src\Application\AdministratorApplication.php:195
16 | Joomla\CMS\Application\AdministratorApplication->doExecute() | JROOT\libraries\src\Application\CMSApplication.php:306
17 | Joomla\CMS\Application\CMSApplication->execute() | JROOT\administrator\includes\app.php:58
18 | require_once() | JROOT\administrator\index.php:32


### System information (as much as possible)
RuntimeException:
Failed to start the session because headers have already been sent by "C:\wamp64\www\j5\libraries\vendor\joomla\application\src\AbstractWebApplication.php" at line 405.

  at C:\wamp64\www\j5\libraries\vendor\joomla\session\src\Storage\NativeStorage.php:454
  at Joomla\Session\Storage\NativeStorage->start()
     (C:\wamp64\www\j5\libraries\src\Session\Storage\JoomlaStorage.php:313)
  at Joomla\CMS\Session\Storage\JoomlaStorage->start()
     (C:\wamp64\www\j5\libraries\vendor\joomla\session\src\Session.php:396)
  at Joomla\Session\Session->start()
     (C:\wamp64\www\j5\libraries\vendor\joomla\session\src\Session.php:327)
  at Joomla\Session\Session->has('user')
     (C:\wamp64\www\j5\libraries\src\Session\Session.php:194)
  at Joomla\CMS\Session\Session->get('user')
     (C:\wamp64\www\j5\libraries\src\Factory.php:372)
  at Joomla\CMS\Factory::getUser()
     (C:\wamp64\www\j5\libraries\src\Session\Session.php:101)
  at Joomla\CMS\Session\Session::getFormToken()
     (C:\wamp64\www\j5\plugins\system\debug\src\Extension\Debug.php:336)
  at Joomla\Plugin\System\Debug\Extension\Debug->onAfterRespond(object(AfterRespondEvent))
     (C:\wamp64\www\j5\libraries\vendor\joomla\event\src\Dispatcher.php:454)
  at Joomla\Event\Dispatcher->dispatch('onAfterRespond', object(AfterRespondEvent))
     (C:\wamp64\www\j5\libraries\vendor\joomla\application\src\AbstractApplication.php:99)
  at Joomla\Application\AbstractApplication->dispatchEvent('onAfterRespond', object(AfterRespondEvent))
     (C:\wamp64\www\j5\libraries\src\Application\CMSApplication.php:351)
  at Joomla\CMS\Application\CMSApplication->execute()
     (C:\wamp64\www\j5\administrator\includes\app.php:58)
  at require_once('C:\\wamp64\\www\\j5\\administrator\\includes\\app.php')
     (C:\wamp64\www\j5\administrator\index.php:32)                

Additional comments

that realy str