IntelliJ Idea mapstruct java: Internal error in the mapping processor: java.lang.NullPointerException

This is a known issue by mapstruct : https://github.com/mapstruct/mapstruct/issues/2215

Solution
Update MapStruct to 1.4.1.Final or later version, see this issue for more details.
Alternative:
You can also add below command at File | Settings (Preferences on macOS) | Build, Execution, Deployment | Compiler | Build process VM options as a workaround.
-Djps.track.ap.dependencies=false