<!--
  Note: if the issue is about documentation or the website, please file it at:
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**Do you want to request a *feature* or report a *bug*?**

bug.

**What is the current behavior?**

```js
  renderer.create(
    <Fragment>
      <Fragment>foo</Fragment>
    </Fragment>
  )
  .toTree()
```

> Invariant Violation: toTree() does not yet know how to handle nodes with tag=10

https://codesandbox.io/s/olky92m01y

**What is the expected behavior?**

returns a tree.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

16.2.0

if (root == null || root.current == null || container == null) {
          return null;
        }
        if (container.children.length === 0) {
          return null;
        }
        if (container.children.length === 1) {
          return toJSON(container.children[0]);
        }
        return container.children.map(toJSON);
      },
      toTree() {
        if (root == null || root.current == null) {
          return null;
        }
        return toTree(root.current);
      },
      update(newElement: React$Element<any>) {
        if (root == null || root.current == null) {
          return;
        }
        TestRenderer.updateContainer(newElement, root, null, null);
      },
      unmount() {
        if (root == null || root.current == null) {
          return;
        }
        TestRenderer.updateContainer(null, root, null, null);
        container = null;
        root = null;