XAML settings are reset at runtime

I am converting my code from Forms to WPF and I am still learning the WPF basics.

I have created a UserControl for a toolbar.  I can add buttons to the toolbar by code without a problem.  I have a List<> property that stores the properties of the button. 

I am using the Get/Set property option to access the List Properties and I can add and remove items in the list through the UserControl property window.   The changes are displayed in the XAML and changes in the XAML show in the properties.

However at runtime the List<> is empty.  It seems to reinitialize the control and does not contain the values that were added in the UserControl properties windows.  Am I missing a step in initializing a list.  I am using the following:

    private List<ButtonItem> listbuttons = new List<ButtonItem>();

Thanks...