Fix React warnings & use after dispose errors

[colin-grant-work]

What it does

This PR fixes a number of React errors related to keys and element hierarchy that appeared in the dev console when a hover was shown.

In addition, when webview was closed, an error would be logged related to an attempt by vscode-messenger to access a property on the now-disposed-of view. That has been fixed.

How to test

1. Open up the dev tools.
2. Open a memory view and show a hover.
3. No React errors should appear in the dev tools console.
4. Close the webview.
5. No errors related to the webview being disposed should appear. Sometimes errors with the text Invalid debug adapter appear, but those appear to be internal to VSCode.

Review checklist

• [x] as an author, I have thoroughly tested my changes and carefully followed the contribution guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the code of conduct

[gbodeen]

Confirmed that I observed no React or other webview errors in dev tools running through the test steps. The code changes make sense for that.