Existing software on your system loaded a system extension signed by “Mattias Nissler” which will be incompatible with a future version of macOS. Contact the developer for support
Keeps coming up after iOS update but gives no other indication of WHAT SOFTWARE and it's impossible to find software on the app store using a developer's name....so the link to About system extensions and macOS given in the last question of this exact same thing doesn't help at all.
MacBook Pro