macosx: don't initiate a modal dialog during a modal session (close #7575)

(cherry picked from commit e608d2aa90a733e48cfc58f9be73f2aea8c75c94)

modules/gui/macosx/open.m

@@ -363,6 +363,10 @@ static VLCOpen *_o_sharedMainInstance = nil;
 
 - (void)openTarget:(int)i_type
 {
+    /* check whether we already run a modal dialog */
+    if ([NSApp modalWindow] != nil)
+        return;
+
     int i_result;
 
     b_autoplay = config_GetInt( VLCIntf, "macosx-autoplay" );