https://codesandbox.io/s/40j50607y0

Rendering a component like:

```js
const Foo = () => <React.Fragment />
```

Throws the error:

> Foo(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

The error is confusing because clearly `Foo` _is_ returning a value. I don't know if we want to support rendering empty fragments, but in any case this error is confusing.

If this isn't allowed it should explicitly call out that a component cannot return a fragment with no children.

cc @gaearon 
}
      }
    }
    if (element._owner) {
      const owner: ?Fiber = (element._owner: any);
      let inst;
      if (owner) {
        const ownerFiber = ((owner: any): Fiber);
        invariant(
          ownerFiber.tag === ClassComponent,
          'Stateless function components cannot have refs.',
        );
        inst = ownerFiber.stateNode;
      }
      invariant(
        inst,
        'Missing owner for string ref %s. This error is likely caused by a ' +
          'bug in React. Please file an issue.',
        mixedRef,
      );
      const stringRef = '' + mixedRef;
      /
      if (
        current !== null &&
        current.ref !== null &&
        typeof current.ref === 'function' &&
        current.ref._stringRef === stringRef
      ) {
        return current.ref;
      }