how to correctly stop uiTimer?

Strange thing - libui `timer` example works just fine, but when converted to Kotlin - crashes on exit:
```
$ gdb timer.exe
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Reading symbols from timer.exe...(no debugging symbols found)...done.
(gdb) r
Starting program: F:\src\kotlin-libui\build\konan\bin\mingw_x64\timer.exe
[New Thread 12232.0x2b0c]
warning: [libui] F:\src\kotlin-libui\libui\windows\alloc.cpp:25:uninitAlloc() You have a bug: Some data was leaked; either you left a uiControl lying around or there's a bug in libui itself. Leaked data:
0x37af40 uiprivTimer

Program received signal SIGTRAP, Trace/breakpoint trap.
0x000007fefcf531f3 in KERNELBASE!DebugBreak ()
from C:\Windows\system32\KernelBase.dll
```
Don't know how to fix this.