Fix inaccurate total_children_count property in modals

[Sacul0457]

Summary

This PR fixes the total_children_count property in modals returning an inaccurate value when
ui.Label is added using the add_item method.
ui.Label should always return 2 items (ui.Label + Component), but rn returns only 1 in _total_count

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)