gridstack.js:153 Uncaught TypeError: Cannot read property 'scrollTop' of null

Subject of the issue

Console error when dragging the rectangle to the top side of the page

gridstack.js:153 Uncaught TypeError: Cannot read property 'scrollTop' of null
at Object.updateScrollPosition (VM282 gridstack.min.js:1)
at HTMLDivElement.t (VM282 gridstack.min.js:1)
at $.(/anonymous function).(anonymous function)._trigger (https://code.jquery.com/ui/1.12.1/jquery-ui.js:697:13)
at $.(/anonymous function).(anonymous function)._trigger (https://code.jquery.com/ui/1.12.1/jquery-ui.js:10264:38)
at $.(/anonymous function).(anonymous function)._trigger (https://code.jquery.com/ui/1.12.1/jquery-ui.js:144:25)
at $.(/anonymous function).(anonymous function)._mouseDrag (https://code.jquery.com/ui/1.12.1/jquery-ui.js:9774:14)
at $.(/anonymous function).(anonymous function)._mouseDrag (https://code.jquery.com/ui/1.12.1/jquery-ui.js:144:25)
at $.(/anonymous function).(anonymous function)._mouseMove (https://code.jquery.com/ui/1.12.1/jquery-ui.js:9427:9)
at $.(/anonymous function).(anonymous function)._mouseMove (https://code.jquery.com/ui/1.12.1/jquery-ui.js:144:25)
at HTMLDocument._mouseMoveDelegate (VM57 jquery-ui.js:9378)

Your environment

gridstack 4.0
lodash 4.17.1
Chrome 66.0.3359.181 64 bits

Steps to reproduce

Example:
https://jsbin.com/fuqomekoja/edit?html,js,output
Just drag one of the rectangles to the top side of the page with the Chrome Console (F12) opened

Expected behaviour

Tell us what should happen

Actual behaviour

Tell us what happens instead