As a solution I can suggest you to bind the SelectedItem property of the ComboBox to a property in your ViewModel this then in the setter of your property you can check whether the new value is null and if it is you can set it to the first item of the collection. You can also set the IsFilteringEnabled
in order to always have the text of the SelectedItem visible. Please check the attached project for the exact approach.
Hope this will work for you.
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