The bug
When a world is already open in another instance of the game, a red warning text is shown, there is no play button and you cannot click on it to focus it:
However, it is possible to select the world with the keyboard:
Using up and down arrow keys
Using tab key in case world is first one in list
This should not be possible, because it enables all buttons and pressing some of them causes errors.
Reproduction steps
Open the game twice
In one instance open a world
In the second instance open the world selection
:info: It should show the warning text for the world which is already openUse the tab key or select another world, then press the up or down arrow keys
❌ It is possible to select the world which is already openPress "Edit"
❌ An error occurs
Can confirm for 1.16 Pre-2. Clicking on the world with a mouse doesn't work, but using tab does.