It seems like there is no binding set.
MyRadComboBox.ItemsSource = MyObservableCollectionOfACustomClass;
The code above just give a source to the ItemsSource, but it doesn't set any binding.
var binding = BindingOperations.GetBindingExpression(MyRadComboBox, RadComboBox.ItemsSourceProperty);
So when the the above is used it is normal that the binding is null.
I have attached a simple project where you can see how when binding is set the binding variable is not null.
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