If you care to try something ...
In the Finder, use the 'Go To' menu for ~/Library/Preferences (the Libary folder in your user's directory) and find any plists with Xcode in their name - I'd expect at least these two:
..and more, perhaps as many as 9 total.
With Xcode not running, delete all of them, empty the trash, then try opening Xcode again by double-clicking in the Finder.
>gives me the error: "Error parsing source tags: Improperly nested tag: editable-code." Duplicating the playground file results in the same error.
And be sure to start using a backup of your projects/playgrounds as a rule.