After installing the newest version getting errors

hello you need to explaine more
maybe you need to read some tutorial
www.flexicontent.org/documentation/tutor...311c02874f047982cf36
flexidemo.com3elles.com/

Nothing in the documentation about the problem I am having. Thanks for trying

no i didn't think your problem is explain in documentation because i think its a setting problem
you need to learn flexicontent more ...
and explain more your problem ?
type used
category template
item template
configuration ... and more
did you have link to see it or post some screenshot

I did not change anything yet, but when I went to create yet another article, now I see "no main category allowed" error.
I do need to learn the extension, I get that, but these errors on a simple text test article are surprising. What am I doing wrong here?

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all these could be because flexisystem plugin is unpublished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...