The bug

When optimizing old worlds (e.g. from Minecraft 1.0) their data is deleted.

Expected would be either, that

• optimizing includes upgrading (if necessary)
• optimizing older / unknown versions is not supported
• the warning text on the "Optimize World" screen is clearer and only shown when optimizing an older world, see MC-181274

How to reproduce

1. Create a world in Minecraft 1.0 or download test 1.0.zip
2. Open the latest version and in the world selection, click "Edit"
3. Select "Optimize world"
   Note that the world now shows the current version as version on the world selection screen
4. Open the word
   You are spawned in the middle of the ocean (you should be in a swamp)