Fix for #10319 - MacOS 2.6 Alpha6 Crash on project load

This issue was not related to the OS and was caused by bug inside export function.
Wrong object id was saved for the cut objects

src/libslic3r/Format/3mf.cpp

@@ -2958,9 +2958,9 @@ namespace Slic3r {
 
         unsigned int object_cnt = 0;
         for (const ModelObject* object : model.objects) {
+            object_cnt++;
             if (!object->is_cut())
                 continue;
-            object_cnt++;
             pt::ptree& obj_tree = tree.add("objects.object", "");
 
             obj_tree.put("<xmlattr>.id", object_cnt);