Handle null children exception

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Expected behavior

If you pass as a child a React Component which could be null, this causes an unexpected TypeError.

Current behavior

Observer is passed the null instance and a TypeError occurs

Steps to reproduce

1. Create an IntersectionObserver as a HOC and pass in a React Component (Class or SFC, returning null) as the only child
2. Observe that the observer attempts to attach to whatever the React Component returns;

Context (environment)

This got past our CI pipeline because TypeScript (correctly) allows null as a valid return type from React.SFC or React.ReactNode. I will try to take a look and provide my own solution, but please also advise if this is due to a limitation within React or otherwise.

• Version: “0.7.3”
• Platform: OSX Debian

[Rendez]

@311289189 see #52

[311289189]

I will give it a shot in the next few days