FormBuilderDropdown can't set itemHeight to null

[joselicht90]

/// 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;

Given the description I should be able to set itemHeight to null, but the type is a non nullable double

[danvick]

I think the API for this attribute has changed in the latest versions of Flutter. We'll fix this ASAP

Note, however, that in this library we have already set the default value of the attribute to kMinInteractiveDimension. So feel free to ignore the attribute and the default value will still be kMinInteractiveDimension