So I tried to implement the tiled map now as importing works but now I get some System.ArgumentNullException when starting the game.
Here is my code if it helps you: https://pastebin.com/ZQZpZV6T
Would be very kind of you if you could help a noob again 
Edit:
I build th MonoGame.Extended stuff again and it startet working. I am not sure what caused it but it works.