Fix T96705: Crash when pressing F3 outside a Blender window if Developer extras

If the mouse is not hovering the window, there is no active region. This is a
valid state, but the UI-list filter operator didn't account for that case.

source/blender/editors/interface/interface_ops.c

@@ -1989,6 +1989,9 @@ static void UI_OT_drop_name(wmOperatorType *ot)
 static bool ui_list_focused_poll(bContext *C)
 {
   const ARegion *region = CTX_wm_region(C);
+  if (!region) {
+    return false;
+  }
   const wmWindow *win = CTX_wm_window(C);
   const uiList *list = UI_list_find_mouse_over(region, win->eventstate);