Add check if no file is open
When I try to init git repo, I receive a "Failed" dialogue box with the following error message:

{
    column = 23;
    line = 64;
    sourceURL = "/Users/diannakane/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Git Sketch Plugin/Git.sketchplugin/Contents/Sketch/Init.cocoascript";
}
