[DocumentViewer] Document specific handlers remain on the screen when changing document type

Steps to reproduce:

1. Open example: https://doc.qt.io/qt-6/qtdoc-demos-documentviewer-example.html from Qt 6.6.0 b1.
2. Build and run the app on Linux desktop.
3. Open any txt file.
4. Open any json file.
5. Open any pdf file.

Expected: Only tools specific for a given document type should be displayed.

Actual: Tools specific to json file remain on the screen when opening pdf files.

Reproducibility:  100%

NOTES: Not reproducible with the example from Qt 6.5.1.