<!--
  Please provide a clear and concise description of what the bug is. Include
  screenshots if needed. Please test using the latest version of the relevant
  React packages to make sure your issue has not already been fixed.
-->

React version: 16.13.1 (also tested with versions down to 16.9.0)
DevTools extension version: 4.8.1 (Firefox), 4.8.0 (Chrome)

## Steps To Reproduce

1. Create a new app using `create-react-app`
2. Replace the contents of `App.js` with this:

```jsx
import React, { Suspense } from "react";

function App() {
  return <Suspense></Suspense>;
}

export default App;
```

## The current behavior
The component tree renders up until the empty `<Suspense>` element, and an error is printed to the console.

Example from a production app:

![image](https://user-images.githubusercontent.com/1078076/87224862-15afcc80-c389-11ea-95da-aa102333befe.png)

Error stack trace (more or less same as in Chrome):
```
Uncaught TypeError: e.child is null
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1029
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1029
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1029
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    Me moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1020
    flushInitialOperations moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1262
    flushInitialOperations moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1257
    o moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:2068
    emit http://localhost:8080/4301/superreports/1359398:1
    emit http://localhost:8080/4301/superreports/1359398:1
    i moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:2075
    y moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:2076
    y moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:2076
    e moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1776
    e moz-extension://3b85dd64-f11d-4480-a0a9-2f6ad211ca37/build/react_devtools_backend.js:1779
```

The line in question is this:
```javascript
// Special case: if Suspense mounts in a timed-out state,
// get the fallback child from the inner fragment and mount
// it as if it was our own child. Updates handle this too.
var u=e.child,c=u?u.sibling:null,s=c?c.child:null;null!==s&&Me(s,a?e:t,!0,r)}else{var f=-1===$?e.child:e.child.child;null!==f&&Me(f,a?e:t,!0,r)}else null!==e.child&&Me(e.child,a?e:t,!0,r);
```
Which points to this location in the source code:
https://github.com/facebook/react/blob/30b47103d4354d9187dc0f1fb804855a5208ca9f/packages/react-devtools-shared/src/backend/renderer.js#L1221

## The expected behavior
I can view the component tree in React DevTools with no error.
idToRootMap.set(id, currentRootID);
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