How to finish SDL screensaver app after preview window closes?

Question

When windows launches my program with /p argument, it initializes SDL window over HWND of that little preview monitor. The problem is, when the preview window closes, my process keeps running. I've tried to catch WM_DESTROY using SDL_SYSWMEVENT, but it didn't work: it catches WM_SETCURSOR, WM_NCHITTEST and some others, but not destroy. How to fix that problem?
Here's my code.

#include <SDL2/SDL.h>
#include <SDL2/SDL_syswm.h>
#include <windows.h>
#include <cstring>
#include <cctype>
#include <cstdlib>
#include <cstdio>

char *lowercase(const char *str) {
    char *ans=new char[strlen(str)+1]; int i;
    for (i=0;str[i]>0;i++) ans[i]=tolower(str[i]);
    ans[i]=0; return ans;
}

int main(int argc,char **argv) {
    if (argc>1&&(strcmp(lowercase(argv[1]),"/c")==0||strcmp(lowercase(argv[1]),"-c")==0)) {
        MessageBox(NULL,"Config should be there, WIP!","Screensaver",MB_OK);
        return 0;
    }
    FILE *f;
    SDL_Init(SDL_INIT_VIDEO);
    int SCREEN_WIDTH,SCREEN_HEIGHT;
    SDL_Window *wnd;
    bool preview;
    if (preview=(argc>2&&(strcmp(lowercase(argv[1]),"/p")==0||strcmp(lowercase(argv[1]),"-p")==0))) {
        wnd=SDL_CreateWindowFrom((HWND)atoi(argv[2]));
        SDL_GetWindowSize(wnd,&SCREEN_WIDTH,&SCREEN_HEIGHT);
    } else {
        SDL_DisplayMode tmp;
        SDL_GetCurrentDisplayMode(0,&tmp);
        SCREEN_WIDTH=tmp.w;SCREEN_HEIGHT=tmp.h;
        wnd=SDL_CreateWindow("",SDL_WINDOWPOS_UNDEFINED,SDL_WINDOWPOS_UNDEFINED,SCREEN_WIDTH,SCREEN_HEIGHT,SDL_WINDOW_SHOWN|SDL_WINDOW_FULLSCREEN);
    }
    SDL_Renderer *rnd=SDL_CreateRenderer(wnd,-1,SDL_RENDERER_ACCELERATED);
    SDL_DisableScreenSaver();
    bool quit=false;
    SDL_Event evt; 
    SDL_EventState(SDL_SYSWMEVENT,SDL_ENABLE);
    while (SDL_PollEvent(&evt)); //clear event queue
    while (!quit) {
        while (SDL_PollEvent(&evt)) switch (evt.type) {
            /* does not work
            case SDL_SYSWMEVENT:  {
                if (evt.syswm.msg->msg.win.msg==WM_DESTROY) quit=true;
                break;
            }
            */
            case SDL_QUIT:
                quit=true;
                break;
            case SDL_KEYDOWN:
            case SDL_MOUSEBUTTONDOWN:
            case SDL_MOUSEMOTION:
                if (!preview) quit=true;
                break;
        }
        SDL_RenderClear(rnd);
        SDL_SetRenderDrawColor(rnd,255,255,255,255);
        SDL_RenderDrawLine(rnd,0,0,SCREEN_WIDTH,SCREEN_HEIGHT);
        SDL_SetRenderDrawColor(rnd,0,0,0,255);
        SDL_RenderPresent(rnd);
        SDL_Delay(20);
    }
    SDL_Quit();
    return 0;
}

Answer 1

Okay, after long research I've found an answer. Thanks to that guy.

if (!IsWindow(previewHwnd)) quit=true;