ICEY - Crash on quitting the game (553640)

Game works fine on Lubuntu 18.04, just quitting the game makes it crash :

Unity Player [version: Unity 5.4.0f3_a6d8d714de6f]

user32.dll caused an Access Violation (0xc0000005)
in module user32.dll at 0033:c532b030.

Error occurred at 2018-08-26_103126.
Z:\mnt\1f44f19b-8b72-44d9-9f51-029a5fed7ef3\SteamLinux\steamapps\common\ICEY\ICEY.exe, run by steamuser.
22% memory in use.
4096 MB physical memory [4096 MB free].
4096 MB paging file [-2048 MB free].
4096 MB user address space [4096 MB free].
Read from location 00000004 caused an access violation.

Context:
RDI: 0x0023e564 RSI: 0x00000018 RAX: 0x00000000
RBX: 0x00000018 RCX: 0x00000000 RDX: 0x10000003
RIP: 0xc532b030 RBP: 0x0023e3f0 SegCs: 0x00000033
EFlags: 0x00010246 RSP: 0x0023e2f0 SegSs: 0x0000002b
R8: 0x00000000 R9: 0x0023e564 R10: 0x00000002
R11: 0xc532ae90 R12: 0x10000003 R13: 0x00000000
R14: 0x0023e564 R15: 0x00000000

Bytes at CS:EIP:
41 8b 5f 04 eb 9e 66 2e 0f 1f 84 00 00 00 00 00

The report identifies ICEY, Unity 5.4.0f3, and an access violation in user32.dll, but mentions no Proton file, test, or entry point. Start by reproducing the crash when quitting ICEY under the reported environment and inspect the resulting crash context. Done means identifying the relevant compatibility failure and confirming that quitting no longer crashes.