Marking class as deobfuscated errors when other classes are already deobfuscated

[Julian Burner]

Similar to #471, but this time not with packages, but with classes. Only happens since #458 Edit: No wait, seems like this has been a problem for a long time now! It's also present in all prominent forks of Enigma (Quilt and Ornithe)

1. Have any class aready in the Deobfuscated Classes panel.
2. Open some other class which is still in the Obfuscated Classes panel.
3. Rename anything other than the class name.
4. Rename the class name.
5. See the error:
   

Now, it seems to not matter which class you choose for step 1, but for step 2 the error doesn't occur with all classes. I'm not sure which patterns this is following, but if you want a reproducible setup, use the latest mapping-io JAR and use net/fabricmc/mappingio/adapter/MappingNsRenamer for step 1 and net/fabricmc/mappingio/MappingWriter for step 2.

If you exclusively use the Mark as deobfuscated context menu option, everything will happen as described above. If you manually rename stuff, in some conditions it also breaks, in some it doesn't. Very weird