[Bug Report] Crash when loading recent simfile while Chart List is visible

[mangoafterdawn]

Describe the bug
ArrowVortex crashes when trying to load a simfile using the File > Recent files dialogue - This only occurs when the Chart List (Chart>Chart List... / Shift+L) is open AND you are attempting to open a simfile with less total difficulty slots than the currently loaded simfile with that dialogue. It also requires that there be a .ogg music file to load.
To Reproduce

Download this simfile-test.zip and skip to step 5 OR:

1. Create two empty folders with any name.
2. Create blank .sm files and any .ogg music files within each folder. (This bug does not occur when the simfile uses a .mp3 music file, OR has an empty music property)
3. In the first .sm file, create 1-2 difficulty slots (Chart>New chart... / Shift+N). Make sure the music property is populated with a .ogg music file
4. In the second .sm file, create at least 3 difficulty slots - the crash does not occur going from a file with 2 => 1 difficulty slots. Make sure the music property is populated with a .ogg music file
5. Open the Chart List (Chart>Chart List... / Shift+L)
6. Open "Simfile1" (or, a simfile with less than 3 slots)
7. Open "Simfile2" (or, a simfile with at least 3 slots)
8. Now that both simfiles have been loaded before, they will appear in File>Recent files. Load the simfile with less than 3 slots again, and observe the crash.

This crash occurs after building the latest commit 764b5b3 on Linux.