Playing around with widgets can be fatal if you meet one that completely eats your CPU. I've found that "dashboard launcher" is such that kind of widget when I dragged it on my plasma 5 desktop. Has anyone faced the same problem or is willing to try?
BTW I needed to kill the plasma session and to edit this file to remove the widget:
./config/plasma-org.kde.plasma.desktop-appletsrc, you need to scroll the file until you find the lines referring to the widget and then remove them. This way it's much more simple and cleaner than removing the whole file or the whole directory as I was used to do.