var searchToAddApplicant = _findToAddFactory.CreatePart();
            searchToAddApplicant.Start();
            //searchToAddApplicant.Parent = this;
            //_windowManager.ShowDialog(searchApplicant, this);
 
            //prepare async dialog operation
            DialogOperationResult<TaggedButtons> asyncOp;
            var findButton = new TaggedButtons("Find" /*todo: replace with translation lookup*/) { IdTag = "FIND" };
            var cancelButton = new TaggedButtons("Cancel" /*todo: replace with translation lookup*/) { IdTag = "" };
            yield return asyncOp = _dialogManager.ShowDialogAsync(
                searchToAddApplicant, new List<TaggedButtons>()
                                          {
                                              findButton,
                                              cancelButton
                                          }, "Find or Add Applicant");

//and ..

internal class TaggedButtons : DialogButton
        {
            private readonly object _value;
 
            public TaggedButtons(object value)
                : base(value)
            {
                _value = value;
            }
 
            public override string ToString()
            {
                return _value.ToString();
            }
 
            public string IdTag { get; set; }
        }
 
 
Making the class public seems to solve the issue. But why didn't I run into this only until after I enabled/disabled OOB?