Fix assignment of mp_stacks_magnificPopup

[acurrieclark]

Attempting to edit any of the bricks in my stacks currently results in a

Uncaught TypeError: Cannot read property 'open' of null

error on the mp_stacks_magnificPopup variable. A little bit of digging in the console revealed that the instance property on $.magnificPopup was always null, but the $.magnificPopup itself has the open() and close() functions we need.

Hopefully this is a genuine fix and not just some bizarre quirk that I am seeing.

[mintplugins]

@acurrieclark Thanks for the PR! I haven't experienced this, but I will test it out asap.