Version

2.3.0

Reproduction link

2.3.0 - https://jsfiddle.net/f0n7jmL7/
2.2.6 - https://jsfiddle.net/eu1q0a8b/

Steps to reproduce

1. Open bug reproduction for 2.3.0 and check console.log - you ca see an error
2. Make change in FormErrors component so it will return empty span tag instead of null value at the end - no error this time and the example works fine

What is expected?

i expect the above example to work as it did in vue@2.2.6

What is actually happening?

Vue throws an error Uncaught TypeError: Cannot set property 'isRootInsert' of null when functional component returns null value instead of vnode.

// EDIT: Also something I noticed in 2.3.0. If you will return undefined instead of null at the end of FormErrors, then our example works again.