Set selectedItem of a ListView, not highlighted

I'm setting in Xamarin Forms the selected item of a ListView but it doesn't highlight.
I can't find an other property in the API like 'HideSelected' of 'Selected' than needs to be set. What did I miss ?

protected override void OnBindingContextChanged() {
        selectedProcess = BrewProcess.GetProcessByID (editBrew.ID); 
        processesListView.SelectedItem = selectedProcess;
}

saveButton.Clicked += (sender, e) => {    
        if (    processesListView.SelectedItem  == null) {
                    DisplayAlert (....

DisplayAlert doesn't show, also check with breakpoint and SelectedItem property is still correct set, it's just not highlighted