serverPrefetch makes render functions return "null"

[arpowers]

Version

1.0.0-beta.29

Reproduction link

https://cannot-do-this-with-ssr.com

Steps to reproduce

1. Import a component that includes the core serverPrefetch property
2. render it
3. the resulting wrapper has HTML/text content of null

What is expected?

The actual HTML for the component

What is actually happening?

It returns null

[arpowers]

The problem is here:

renderer.renderToString(vm, function (err, res) {
    if (err) {
      throw err
    }
   
    renderedString = res
  })
  return renderedString

the serverPrefetch makes the callback take slightly longer, but this returns renderedString before it calls