Fix crash when open default template

As soon you select the default template, Blender crash on Winodws.

This was introduced in commit {0b626703}

source/blender/editors/interface/interface_region_popup.c

@@ -554,7 +554,7 @@ static void ui_popup_block_remove(bContext *C, uiPopupBlockHandle *handle)
   CTX_wm_region_set(C, ctx_ar);
 
   /* reset to region cursor (only if there's not another menu open) */
-  if (BLI_listbase_is_empty(&sc->regionbase)) {
+  if ((ctx_sa != NULL) && (BLI_listbase_is_empty(&sc->regionbase))) {
     ctx_sa->flag |= AREA_FLAG_CURSOR_UPDATE;
   }