Shutter is running:
How do I open its main window?
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Well, as you can see, there is not such an option in the menu that you showed in the image. Even so, you can select Selection or Section from that menu and after press Esc key; you will remain with the Shutter's main window in the front.
Another way is to press Alt+F2 and type shutter followed by Enter.
Personally, I keep Shutter in the Launcher and every time when I need it, I click it.