An update to this…
I’ve since began using other widgets (vtkDistanceWidget, vtkAngleWidget) in addition to vtkCaptionWidget and the line I added above seems to cause other issues on application shutdown with those widget types involved. Therefore, I had to remove that line, and after much trial and error I’ve found the best thing to do on application shutdown is to simply call the Delete() method on all the widgets before deleting the RenderWindowInteractor, RenderWindow and Renderer. This seems to work fine for all widget types I’ve tried - no crashes.