"Critical Error: Object from PIW still referenced" on game shutdown

Do you have any idea have could cause this? I am avoind blueprints as much as I can and only using them for UMG / Slate Widgets, so I am very unexperienced with it. Do I have to manually delete something? I followed Rama’s tutorial on creating scrollable button lists. Do you need some pics of the Blueprints?

Basically, I have a blueprint with a collection of “Noblemans” and when I click one, a widget is opened with details about this nobleman. Only when I open this widget, the error occurs. I pass the Nobleman reference to each widget.