After installing the newest version getting errors

Hello these are strange errors
all these could be because flexisystem plugin is unpublished
Please check that plugin is published, and then maybe clear cache too

I have installed the latest Joomla as well as the latest Flexicontent. There is a major issue. When I click on an item to view, or to try and create a new item, I get:

0 Call to undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin()

I have looked at the call stack and the frontend works fine, it is just that since migrating, I cannot add any content. That is kind of pointless to have a site I cannot update. The only other component that doesn't work on the site is the Media manager which gives an error of: 

Property 'mime_type' of argument 'item' of event onFetchMediaItems has a wrong item. Valid: string

This too feels linked, because the fileinfo ini is working according to the server. 

Can somebody please help, I really don't want to have to rebuild the whole content with a fresh install... 

no your error is an old depreciate code "isadmin" but i didn't think it flexicontent
please set error_reporting to maxium and debug to yes in joomla configuration
you will see more error information

This is what the debug says:
Call stack
# Function Location
1 () JROOT/plugins/editors-xtd/upagebutton/upagebutton.php:17
2 PlgButtonUpagebutton->__construct() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:234
3 Joomla\CMS\Application\CMSApplication->loadPluginFromFilesystem() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:157
4 Joomla\CMS\Application\CMSApplication->loadExtension() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:96
5 Joomla\CMS\Application\CMSApplication->bootPlugin() JROOT/libraries/src/Plugin/PluginHelper.php:221
6 Joomla\CMS\Plugin\PluginHelper::import() JROOT/libraries/src/Plugin/PluginHelper.php:181
7 Joomla\CMS\Plugin\PluginHelper::importPlugin() JROOT/libraries/src/Editor/Editor.php:309
8 Joomla\CMS\Editor\Editor->_loadEditor() JROOT/libraries/src/Editor/Editor.php:170
9 Joomla\CMS\Editor\Editor->display() JROOT/plugins/flexicontent_fields/textarea/tmpl/field_default.php:45
10 include() JROOT/plugins/flexicontent_fields/textarea/textarea.php:532
11 plgFlexicontent_fieldsTextarea->onDisplayField() JROOT/components/com_flexicontent/classes/helpers/flexiutilities.php:428
12 FLEXIUtilities::call_FC_Field_Func() JROOT/components/com_flexicontent/classes/flexicontent.fields.php:505
13 FlexicontentFields::getFieldFormDisplay() JROOT/administrator/components/com_flexicontent/views/item/view.html.php:693
14 FlexicontentViewItem->_displayForm() JROOT/administrator/components/com_flexicontent/views/item/view.html.php:49
15 FlexicontentViewItem->display() JROOT/administrator/components/com_flexicontent/controllers/items.php:1470
16 FlexicontentControllerItems->edit() JROOT/libraries/src/MVC/Controller/BaseController.php:672
17 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/administrator/components/com_flexicontent/admin.flexicontent.php:264
18 require_once() JROOT/administrator/components/com_flexicontent/flexicontent.php:22
19 require_once() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:71
20 Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:73
21 Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:355
22 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:143
23 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:186
24 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:294
25 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:61
26 require_once() JROOT/administrator/index.php:32

hello check this plugin
1 () JROOT/plugins/editors-xtd/upagebutton/upagebutton.php:17
i think its not j4 compatible