TypeError when using React: Cannot read property 'firstChild' of undefined

Question

Sometimes, when using React libraries, such as react-router, I get this error:

Uncaught TypeError: Cannot read property \'firstChild\' of undefined

Answer 1

Using require('react') and require('React') inconsistently causes this problem as well, even if you only have one version of React.

https://www.npmjs.com/package/gulp-browserify didn't have this problem. Once I stopped using gulp-browserify and switched to watchify+browserify, Uncaught TypeError: Cannot read property 'firstChild' of undefined started happening.