SDL: X11TK: fix late null-check causing segfault

SDL Commits
1 
From e70f1bfc29aa0aa9a4c066725df48ab336067d73 Mon Sep 17 00:00:00 2001
From: Igor <[EMAIL REDACTED]>
Date: Fri, 1 May 2026 18:01:13 -0300
Subject: [PATCH] X11TK: fix late null-check causing segfault

The code is using `controls.window` before checking if it isn't null.

I found this bug by accident when I tried to run `SDL_ShowSimpleMessageBox`.

It first tried using Wayland with `zenity`, but since I don't have `zenity`, it fallbacked to `X11_ShowMessageBoxImpl`.

For some reason it couldn't create a window, maybe something related to XWayland, so `controls.window` was `NULL`.
---
 src/video/x11/SDL_x11messagebox.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/video/x11/SDL_x11messagebox.c b/src/video/x11/SDL_x11messagebox.c
index 251079a5d53ed..eff7aebe52458 100644
--- a/src/video/x11/SDL_x11messagebox.c
+++ b/src/video/x11/SDL_x11messagebox.c
@@ -250,11 +250,11 @@ static bool X11_ShowMessageBoxImpl(const SDL_MessageBoxData *messageboxdata, int
 #else 
     controls.window = X11Toolkit_CreateWindowStruct(parent_window, NULL, SDL_TOOLKIT_WINDOW_MODE_X11_DIALOG, colorhints, false);
 #endif
-    controls.window->cb_data = &controls;
-    controls.window->cb_on_scale_change = X11_OnMessageBoxScaleChange;
     if (!controls.window) {
         return false;
     }
+    controls.window->cb_data = &controls;
+    controls.window->cb_on_scale_change = X11_OnMessageBoxScaleChange;
 
     /* Create controls */
     controls.buttonID = buttonID;