Thanks, I'll look into this. There are further issues with the combobox, it certainly needs attention.

I had to play around quite a while to find out when the problem you reported actually happens. It turns out that you need to have a filter in place to reproduce the issue.

Please in the future make sure you describe your problem in more detail, to avoid the developer having to spend extra time to find out what you already know, but chose not to share (the one about the filter), or not specifically enough (such as "access" and "data" in your description). Thanks!
https://www.slicer.org/wiki/Documentation/Nightly/Report_a_problem

Sorry about not being specific enough about accessing the widget. I didn't know I needed to be more specific than "before any data is loaded". In my testing, most qMRMLSubjectHierarchyComboBoxes had filters set, so I didn't notice it is a requirement for the bug to surface.

But you could have also asked for more information about reproducing, you didn't have to spend to much time re-creating it.

Hi Dzenan, I believe I fixed the issue. This was a tough one!
https://github.com/Slicer/Slicer/commit/cc182eae3c92230be87a27b43d98a4d3dfaffcf7

Please see if it works tomorrow. If so, please close the ticket.

That seems to have fixed it. Thanks Csaba!