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React version: 16.12.0

## Steps To Reproduce

1. Create an app with a `Proxy` function that has a `get` method that returns a function stored in Context
2. Attempt to look at Provider component in devtools

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Link to code example:
https://codesandbox.io/s/stoic-jepsen-7jjfn?file=/src/App.js
https://7jjfn.csb.app/
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## The current behavior
When using the devtools to inspect the `Context.Provider`, devtools throw an error:
```
Uncaught DOMException: Failed to execute 'postMessage' on 'Window': function () {
        return null;
      } could not be cloned.
```
Components using the context do not show info in devtools and show "Loading..." instead.

## The expected behavior
No error is thrown.

## Possibly Related
#16691
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