When MainView.xaml is opened, the designer throws NullReferenceException because the SelectedOutlookSection is not initialized.

Workaround: If SelectedOutlookSection is null, then initialize it to point to the first item from the collection of outlook sections:

public OutlookSection SelectedOutlookSection
{
    get
    {
        if (this._selectedOutlookSection == null)
        {
            this._selectedOutlookSection = this.OutlookSections.FirstOrDefault();
        }

        return this._selectedOutlookSection;
    }
  ...
}

The fix is available in R1 2017 SP1.