I'm sorry to say that but your fix didn't help. Latest src.

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
  * frame #0: 0x00007fff687bf32a libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x00007fff6887fe60 libsystem_pthread.dylib`pthread_kill + 430
    frame #2: 0x00007fff68746808 libsystem_c.dylib`abort + 120
    frame #3: 0x00007fff659ab458 libc++abi.dylib`abort_message + 231
    frame #4: 0x00007fff6599c8bf libc++abi.dylib`demangling_terminate_handler() + 262
    frame #5: 0x00007fff674d9a57 libobjc.A.dylib`_objc_terminate() + 96
    frame #6: 0x00007fff659aa887 libc++abi.dylib`std::__terminate(void (*)()) + 8
    frame #7: 0x00007fff659ad1a2 libc++abi.dylib`__cxxabiv1::failed_throw(__cxxabiv1::__cxa_exception*) + 27
    frame #8: 0x00007fff659ad169 libc++abi.dylib`__cxa_throw + 113
    frame #9: 0x00007fff674d7bcc libobjc.A.dylib`objc_exception_throw + 350
    frame #10: 0x00007fff2ea38e36 CoreFoundation`-[NSObject(NSObject) doesNotRecognizeSelector:] + 132
    frame #11: 0x00007fff2e91e190 CoreFoundation`___forwarding___ + 1427
    frame #12: 0x00007fff2e91db68 CoreFoundation`__forwarding_prep_0___ + 120
    frame #13: 0x00000001004963fe ide`Coco_PaintCh(cgcontext=0x0000000102b13a10, type=7, value=1, state=0) at ChCocoMM.mm:99:2
    frame #14: 0x0000000100440367 ide`Upp::CocoImg(bg=(color = 1073741824), type=7, value=1, state=0) at ChCoco.cpp:47:2
    frame #15: 0x00000001004407ea ide`Upp::CocoColor(k=1, bg=(color = 1073741824)) at ChCoco.cpp:69:18
    frame #16: 0x000000010043e70e ide`Upp::ChHostSkin() at ChCoco.cpp:136:20
    frame #17: 0x0000000100a2979f ide`Upp::Ctrl::ReSkin() at Ctrl.cpp:951:3
    frame #18: 0x0000000100a29dc0 ide`Upp::Ctrl::SetUHDEnabled(set=true) at Ctrl.cpp:794:2
    frame #19: 0x0000000100a6eb05 ide`Upp::CocoInit(argc=1, argv=0x00007ffeefbff568, envptr=0x00007ffeefbff578) at CocoApp.mm:99:2
    frame #20: 0x00000001000ee90c ide`main(argc=1, argv=0x00007ffeefbff568, envptr=0x00007ffeefbff578) at main.cpp:178:1
    frame #21: 0x00007fff68677cc9 libdyld.dylib`start + 1
    frame #22: 0x00007fff68677cc9 libdyld.dylib`start + 1

This is what I've found: AppKit macOS 10.15.6 to 11.0 API Differences
[NSAppearance performAsCurrentDrawingAppearance:] was added in 11.0. It is not available in 10.15.
It would be great to be able to detect version of API at run time (or at compile time) and adapt code accordingly ...