[gui] Artboard editor crash on opening edit modal

[ashifolfi]

Error: artboard/artentry.lua:75: attempt to index global 'openartfile' (a nil value)
stack traceback:
        [love "boot.lua"]:345: in function '__index'
        artboard/artentry.lua:75: in function 'DoEditModal'
        artboard/artentry.lua:118: in function 'DrawListBlock'
        artboard/artboard.lua:68: in function 'drawBlocks'
        artboard/artboard.lua:136: in function 'draw'
        artboard/init.lua:55: in function 'renderContents'
        uitypes/panel.lua:21: in function 'render'
        maingui/init.lua:63: in function 'render'
        main.lua:51: in function 'draw'
        [love "callbacks.lua"]:168: in function <[love "callbacks.lua"]:144>
        [C]: in function 'xpcall'

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