<!--
  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 Developer Tools: 4.4.0 f749045a5 (1/3/2020) from chrome webstore
Chromium: 81.0.4024.0 snapshot
Ubuntu: 18.04

## Steps To Reproduce

1. Open chrome with React Developer Tools installed
2. Open developer console
3. In console settings (cogwheel in console's top right corner)  check "Preserve log" checkbox (to make sure that the log is not overwritten on navigation)
4. Navigate to a React-enabled website, e.g. `https://reactjs.org`
5. Navigate to a plain-text page, such as `https://reactjs.org/robots.txt`

## The current behavior

Error is printed in console:

```
backend.js:32 Uncaught TypeError: Cannot read property 'sub' of undefined
    at g (backend.js:32)
    at e (backend.js:8)
g @ backend.js:32
e @ backend.js:8
postMessage (async)
a @ contentScript.js:1
117 @ contentScript.js:1
n @ contentScript.js:1
(anonymous) @ contentScript.js:1
(anonymous) @ contentScript.js:1

```
where `backend.js` is a link to `chrome-extension://fmkadmapgofadopljbjfkapdkoienihi/build/backend.js`

`fmkadmapgofadopljbjfkapdkoienihi` is an ID of React Developer Tools: I verified by opening `chrome://extensions/` and performing page search for this ID.


## The expected behavior

No errors

## Notes

It's a convoluted usecase, but I thought it may help to catch bugs for more important ones. 

I am not sure whether it's plain-text-ness of the page that is important, but that's how you can reproduce it.

The hypothesis is that dev tools do not expect HTML tree to disappear on navigation, or either extension enters a state where it cannot digest the plain text pages (and it probably shouldn't try).


<sup>
P.S. Thanks for the refreshed extension, it makes dev experience so wonderful! ❤️ 
</sup>
return lazyNativeStyleEditorValidAttributes;
            },
            set(value) {
              lazyNativeStyleEditorValidAttributes = value;
              initAfterTick();
            },
          }: Object),
        );
      }
    }
  };
  function handleClose() {
    if (__DEBUG__) {
      debug('WebSocket.onclose');
    }
    if (bridge !== null) {
      bridge.emit('shutdown');
    }
    scheduleRetry();
  }
  function handleFailed() {
    if (__DEBUG__) {
      debug('WebSocket.onerror');
    }
    scheduleRetry();
  }
  function handleMessage(event) {
    let data;