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**Do you want to request a *feature* or report a *bug*?**

bug

**What is the current behavior?**

Given code like this:
```jsx
<Suspense fallback={...}>
  {this.state.render && <LazyComponent />}
</Suspense>
```
my `LazyComponent` should download and render when I run `this.setState({ render: true })`. Instead, I get an internal React error about setting property `null` of `return`.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:**

https://jsfiddle.net/cq9hfwe8/1/. Open the console to see the error message.

**What is the expected behavior?**

The combination of `Suspense` and `lazy` defer downloading and rendering my component until I set a state variable to `true`.

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

Tried on Windows in Chrome 71, React 16.6.1.

}
      const root = ReactTestRenderer.create(<App />, {
        unstable_isConcurrent: true,
      });
      
      expect(root).toFlushAndYield(['Suspend! [Step: 1]', 'Loading...']);
      jest.advanceTimersByTime(1000);
      expect(ReactTestRenderer).toHaveYielded(['Promise resolved [Step: 1]']);
      expect(root).toFlushAndYield(['Step: 1']);
      expect(root).toMatchRenderedOutput('Step: 1');
      
      instance.setState({step: 2});
      expect(root).toFlushAndYield(['Suspend! [Step: 2]', 'Loading...']);
      jest.advanceTimersByTime(500);
      expect(root).toMatchRenderedOutput('Loading...');
      
      instance.setState({step: 3});
      expect(root).toFlushAndYield(['Suspend! [Step: 3]']);
      expect(root).toMatchRenderedOutput('Loading...');
      jest.advanceTimersByTime(1000);
      expect(ReactTestRenderer).toHaveYielded([
        'Promise resolved [Step: 2]',
        'Promise resolved [Step: 3]',
      ]);
      expect(root).toFlushAndYield(['Step: 3']);
      expect(root).toMatchRenderedOutput('Step: 3');
    });
    it('does not remount the fallback while suspended children resolve in sync mode', () => {
      let mounts = 0;
      class ShouldMountOnce extends React.Component {