saveFile() tries to call a non-function callback

In grapheditorxblock/src/js/grapheditorxblock.js we have a saveFile() function that is supposed to be called with a callback functions as a parameter. Apparently the check we do on that does not suffice:

if (typeof callback != "undefined")
    callback();

This can result in the following JavaScript error: TypeError: callback is not a function

Not sure if this is relevant: I consistently get that error (hidden in the console of course) when saving in the studio view in my local development environment while testing the graphxblock stand-alone. I have not yet tested other situations. Even with that error message, saving seems to work fine, so this is a low-priority issue.