If null, then the menu item heights will vary according to each menu item's intrinsic height.
The default value is kMinInteractiveDimension, which is also the minimum height for menu items.
If this value is null and there isn't enough vertical room for the menu, then the menu's initial scroll offset may not align the selected item with the dropdown button. That's because, in this case, the initial scroll offset is computed as if all of the menu item heights were kMinInteractiveDimension.
final double itemHeight