Three points from discussion in https://github.com/reactjs/react-redux/issues/756:

* Due to https://bugs.chromium.org/p/chromium/issues/detail?id=701371, environments like CodeSandbox show `Error: null` in our “React caught an error” warning. Let’s consider skipping both as they’re not very useful and look like a bug. `Error: null` looks like a separate error and draws attention away from the real error just above.

* In any case we should probably not print `The error was thrown at:` if we don’t have a JS stack.

* There is a related issue. What do we pass to error boundary if we caught `null`? It seems that most error boundaries will intuitively do something like `if (this.state.error)` or read `error.message`, which `null` will break. There are other falsy values too like `''` and `undefined`. My proposal is we always wrap primitives into our own `Error` objects that we pass to the boundary.
<li>Open the browser DevTools</li>
            <li>Make sure "Pause on exceptions" is enabled</li>
            <li>Make sure "Pause on caught exceptions" is disabled</li>
            <li>Click the "Trigger error" button</li>
            <li>Click the reset button</li>
          </TestCase.Steps>
          <TestCase.ExpectedResult>
            The DevTools should pause at the line where the error was thrown, in
            the BadRender component. After resuming, the "Trigger error" button
            should be replaced with "Captured an error: Oops!" Clicking reset
            should reset the test case.
          </TestCase.ExpectedResult>
          <Example error={new Error('Oops!')} />
        </TestCase>
        <TestCase title="Throwing null" description="">
          <TestCase.Steps>
            <li>Click the "Trigger error" button</li>
            <li>Click the reset button</li>
          </TestCase.Steps>
          <TestCase.ExpectedResult>
            The "Trigger error" button should be replaced with "Captured an
            error: null". Clicking reset should reset the test case.
          </TestCase.ExpectedResult>
          <Example error={null} />
        </TestCase>
      </FixtureSet>
    );
  }
}