[2.3.0] Uncaught TypeError: Cannot set property 'isRootInsert' of null

[sqal]

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.

[gebilaoxiong]

HI, I think this problem is caused when creating vnode

https://github.com/vuejs/vue/blob/dev/src/core/vdom/create-element.js#L102

in 2.2.6:

  if (vnode) {
    if (ns) { applyNS(vnode, ns); }
    return vnode
  } else {
    return createEmptyVNode()
  }

because of your component FormErrors return null so...

You can temporarily output an empty element to solve it return undefined