loadSVGFromURL error

[AlexSergey]

Version

2.3.6

Test Case

When we try to set incorrect URL we get empty xml variable. It call callback with null, but we need prevent function, something like return false, etc. Because after callback(null) this code doesn't dying and try to call parseSVGDocument

http://joxi.ru/gmvpaj4TL85vPA

Steps to reproduce

1. Go to http://fabricjs.com/kitchensink
2. Set "Execute"
3. Paste the code:
   fabric.loadSVGFromURL('https://www.cleverbrush.com/f/mu27s/w5jspy20v+.svg', objects => {
   console.log(objects);
   });

Expected Behavior

We need to get only null in console

Actual Behavior

We get also error - Cannot read property 'documentElement' of null