In Design > Images right click to Add Images and in the pane which opens CTRL click two folders holding images, then click Open. Bootstrap Studio freezes. I attempted this to see if I could create folders and import their images in one action rather than create the folders in Bootstrap Studio and then import images into them.
Sorry that you've run into problems! Do you have any idea how many images there were in those two folders?
From memory, there would have been around 10 or 20.