[WC] Cascading combos selection persistence

[IMinchev64]

Three issues are identified in the grid cascading combos sample:

1. Persistent dropdown content: When a country/region is deselected, the region/city dropdown should ideally be emptied or reset. However, the region dropdown remains populated until the user clicks elsewhere on the screen.

2. Error selecting from unpopulated dropdown: If the country/region is deselected and the region/city dropdown, which should be empty, still allows selection, it leads to an error stating the selected member is undefined.

3. Selection persistence: After selecting a country/region and a region/city, if the country/region is then deselected, the region/city selection remains.