Trying to open old worlds (in Region format?) without version information in snapshots does not show warning screen

The bug

Normally, when you try to open an old world an orange exclamation mark is shown in the world selection and a warning screen is shown recommending to make a backup.

However, this is not the case when opening worlds, which have been created before version information was stored in the world files and are in the Region format (< 1.2.1 ?), in a snapshot version (this appears to work correctly when opening them in the release 1.15.2 though).

Reproduction steps

1. Create a world for example in Minecraft 1.0, or download test 1.0.zip
2. Try to open the world in the latest snapshot
   No warning screen is shown (also the game might crash, see MC-180372)