Playground Markdown Parser Bug

Number:rdar://36184289 Date Originated:12/21/2017
Status:Open Resolved:
Product:Swift Playgrounds Product Version:Xcode Version 9.2 (9C40b)
Classification:Bug Reproducible:Yes
When a Markdown comment block is not closed properly, Xcode crashes when the user attempts to toggle off the `Render Documentation` option.

Steps to Reproduce:
Create a Markdown comment block as such

      comments here

 >> Notice how the block is NOT closed off properly

Toggle the `Render Documentation` option repeatedly. 

Expected Results:
Xcode should provide a warning and or suggest a fix.

Actual Results:
The application crashes when the user attempts to toggle `Render Document` from On to Off.

Xcode Version 9.2 (9C40b)

MacBook Pro (13-inch, 2016, Two Thunderbolt 3 ports)


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