RowHeight of a grouped list

Hi,
in my application I cannot use the RowHeight Property of a grouped list. It is always -1.

My Code in which I needed the Property is:
this.<ListView>.HeightRequest = ( (<AnyViewModel>)this.BindingContext ).<AnyObservableCollection>.Sum ( a => a.Count ) * this.<ListView>.RowHeight;

this is a phrase from my code-behind and it is set to BindingContextChangedEvent and I am trying to rearrange the HeightRequest Property with a product from my elements and the rowheight...

<ListView> is a grouped List...
<AnyObservableCollection> is the Itemssource