Issue with plt:framework-pref:drracket:most-recent-lang-line

[sorawee]

Steps to reproduce:

1. Set plt:framework-pref:drracket:most-recent-lang-line to #lang r6rs and close DrRacket. E.g. by creating a new buffer, change the lang line, save it to a file, and close DrRacket.
2. Open DrRacket. #lang r6rs should be loaded by default, with an error

   r6rs: must contain a `library' form (for a library) or start with `import' (for a top-level program) in: (#%module-begin)
   

3. Open any other file that is not a #lang r6rs. The error about

   r6rs: must contain a `library' form (for a library) or start with `import' (for a top-level program) in: (#%module-begin)
   

   persists, and incorrectly highlighted in this file.