I'm sorry, but that's not Pantheon Calculator - If your system is fully updated, you most likely have two "Calculators". The one shown on your screenshot is gnome-calculator (or gcalctool, I'm not 100% sure about the naming) which was shipped with Freya before we introduced Pantheon Calculator. You can remove that package from your system. If the same thing happens with Pantheon Calculator, feel free to post to this bug again.
I'm sorry, but that's not Pantheon Calculator - If your system is fully updated, you most likely have two "Calculators". The one shown on your screenshot is gnome-calculator (or gcalctool, I'm not 100% sure about the naming) which was shipped with Freya before we introduced Pantheon Calculator. You can remove that package from your system. If the same thing happens with Pantheon Calculator, feel free to post to this bug again.