<Typography sx={{ minWidth: 300 }}>
  Selected item: {selectedItem == null ? 'none' : selectedItem.label}
</Typography>
<Box sx={{ minHeight: 352, minWidth: 250 }}>
  <RichTreeView
    items={MUI_X_PRODUCTS}
    apiRef={apiRef}
    selectedItems={selectedItem?.id ?? null}
    onSelectedItemsChange={handleSelectedItemsChange}
  />
</Box>