ToolbarItem Icon Property - Blue Box instead of Icon Image in NavigationBar

Hello,

can anyone post an example of the toolbar item icon implementation?
I only see a clickable blue box with the dimensions of my icon in the navigation bar when I implement it in XAML or in CodeBehind like shown below.

<ContentPage>
<ToolbarItem Name="Icon" Icon="filter.png" Order="Default"/>
</ContentPage>

When I set the Order property to "Secondary" then it works and I see the icon and the text.. but it is in a second bar then and not in the main navigation bar. Only showing a text without an icon in the navigation bar works too.

Thank You