After upgrading to the version 2020.3 of Idea i get a NullPointerException for the mapping processor. If anybody has a clue... Thank you!
The solution is to update MapStruct to 1.4.1.Final or later version, see this issue for more details.
You can also add -Djps.track.ap.dependencies=false at File | Settings (Preferences on macOS) | Build, Execution, Deployment | Compiler | Build process VM options as a workaround.