[BUG] Window becomes invisible undead zombie when closed through "X" in top right corner

[TheKins]

Observation Environment

Piccu Engine v1.1.1 (reissue)
Windows 10 Home

Reproduction Steps

1. Run Piccu Engine in Windowed mode.
2. Close the window through the standard windows close control (the X in the top right corner, on the titlebar)

Expected Behavior

The window closes and the process is gracefully terminated.

Observed Behavior

The window closes, but the process remains active and continues to play audio, even without a window, until it is terminated manually in the Task Manager.