Native Photo Picker does not react to tapping "Cancel" or selecting a photo. Hence, there's no way to close the picker.
The implementation is as simple as

import QtCore
import QtQuick
import QtQuick.Controls
import QtQuick.Dialogs

Window {
...
         FileDialog {
             id: fileDialog
             currentFolder: "file:assets-library://"
             onAccepted:
             {
                 image.source = selectedFile
             }
         }
...

Known issue
Only happens for the first time after the app's installation.
-> Need to delete and deploy in order to reproduce again.
Works fine in Qt5.15.11 with labs FileDialog.

How to reproduce
1. Please use Qtbug109120.zip .
2. Run and click the button to open the native image picker.
3. Tap one photo so that it will show a message dialog as below.

4. Select "Select Photos".
5. Tapping "Cancel" or a photo does nothing.

Note : Photos/Album can be switched, and also it's possible to view albums so it's not completely stuck.