Flutter: NoSuchMethodError: The method ' / was called on null解决方案

Flutter出现以下错误：

NoSuchMethodError: The method ' / was called on null.

Receiver: null

Tried calling: /(1334.0)

解决方案：在调用前先对ScreenUtil.instance初始化宽高，如下：

ScreenUtil.instance = ScreenUtil(width: 750, height: 1334)..init(context);