Crash when using `swap_main` on a floating window

[randoragon]

Steps to reproduce:

1. Spawn 2 tiled windows.
2. Make one of them floating.
3. swap_main with the floating window focused.
4. Close the (still focused) floating window.
5. Spawn a new window. Spectrwm should crash.

spectrwm 3.5.1 Build: 3.5.1
Arch Linux
My configuration