Bug with React Strict Mode

[SanjoSolutions]

Hi, I have created a fix to make the version 7.2.x work with React Strict Mode enabled.

This fixes jquense/react-big-calendar#2434 and jquense/react-big-calendar#2631.

You can find it here: https://github.com/SanjoSolutions/uncontrollable/tree/fix/react-strict-mode

I wasn't able to make an appropriate pull request because dd40d92600566b0af14e66e0942b3066e2f62528 (7.2.1) was used as a base. You can merge the branch into a 7.2.x branch and release it as 7.2.2.