looking for flutter pro to update project

The project should be published at apple. It is a music player. It looks like the music player is not worling any more. We get an error while starting the app in the simulator. See picture. I guess it has to do with null safety. As I am no programmer, I could ony guess. We could give you access to our development environment via teamviewer or send you the code.
If this prohects works we would like to add in app buys to the app.