How do I stop data binding error when deselecting item in combobox?

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C#

WPF

combobox

data-binding

I have a screen that shows what is in effect a master-detail view. The master is a list of projects. The detail part of the view has a ComboBox that shows the name of the Project Manager for the selected project. The ComboBox list is bound to a list of names supplied by a different class entirely.

Everything works fine until the I hit a button that is supposed to deselect everything on the screen - i.e. put the screen back into the state it was first loaded. When I set the SelectedIndex property of the ComboBox to -1, I get the following error:

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.ItemsControl', AncestorLevel='1''. BindingExpression:Path=HorizontalContentAlignment; DataItem=null; target element is 'ComboBoxItem' (Name=''); target property is 'HorizontalContentAlignment' (type 'HorizontalAlignment')

Does anyone have any idea what may be going wrong? Have to admit I am in the dark how to debug this. I seem to have tried everything.

Kind wishes ~ Patrick

What I have tried:

I have tried setting the SelectedItem to null at the same time as setting SelectedIndex to -1.

I have tried not setting the SelectedIndex at all, which gives no error but leaves the box displaying the name from the last project that was selected.

I have tried adding an empty string to the list of names that the ComboBox is bound to.

And I am sure I have tried about 50 other things I can't even remember.