[Github] "Edit on HackMD" button can't display while the document is empty.

[splitline]

And the following is the error message show in console when I open this page.

content.js:80 TypeError: Cannot read property '1' of null
    at t (content.js:80)
    at content.js:80
    at e.trigger (content.js:80)
    at e.<anonymous> (content.js:80)
    at content.js:80
    at Object.next (content.js:80)
    at content.js:80
    at new Promise (<anonymous>)
    at r (content.js:80)
    at e.triggerAsync (content.js:80)