failed-assertion when showing macOS configure sheet

[DavidLudwig]

Steps to reproduce:

1. build for, and run on, macOS + Metal
   1. open FallingStuff.xcodeproj (in root directory)
   2. build and run the FallingStuff_OSX target
2. observe that the app's window appears
3. if the window is not active, make it so
4. press the 'C' key

Expected result: configure sheet drops down

Observed result: failed assertion, perhaps a crash, and definitely no configure sheet