Fix null error on removing children

[jakeane]

Discovered issue when children are removed from the component. Results in this error

Uncaught TypeError: Cannot read property 'offsetTop' of null
    at _default._this.handleVerticalScroll (component.js:72)
    at _this.handleScroll (component.js:94)

Now null checks are performed in the problematic sections. This will avoid an error being raised unnecessarily.

Also, I removed the engine requirements, as they seem inconsequential.