修复useUnmountedRef与并发模式的不兼容性

[otakustay]

参考 #1021

这东西需要并发模式，没办法测试，所以我没有增加相应的UT

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[otakustay]

一定需要签CLA吗？我的工作身份恐怕并不方便我这么干，如果必须签的话，请关闭这个PR，代码很简单可以由他人提交修复。

[brickspert]

LGTM