If you specifically need the old nullDC APK for testing or nostalgia, be aware it is quite buggy on modern Android versions:
The closest working link you need is actually the Google Play Store page for Redream or RetroArch (Flycast core) .
The emulator software itself is usually legal (open source). However, downloading APKs from unofficial sources violates Google's security policies and exposes you to malware. It is safer to use legal, Play Store alternatives.
If you specifically need the old nullDC APK for testing or nostalgia, be aware it is quite buggy on modern Android versions:
The closest working link you need is actually the Google Play Store page for Redream or RetroArch (Flycast core) . nulldc apk android link
The emulator software itself is usually legal (open source). However, downloading APKs from unofficial sources violates Google's security policies and exposes you to malware. It is safer to use legal, Play Store alternatives. If you specifically need the old nullDC APK