useHeights内htmlElement为null兼容

[snowing-x]

htmlElement为null情况下，获取offsetHeight会报错，希望可以兼容一下

[yoyo837]

来个复现到antd上提交个问题？

[lift46252]

using of deprecated findDOMNode throws error on component
and as i see findDOMNode is not needed here

[lift46252]

PR #282 to fix #259

[lift46252]

@zombieJ #282 (comment)