Empty webview tab on reopen of VS Code

[ScottCarda-MS]

Describe the bug

When you open a VS Code instance that had more than one Q# webview tab open from the last time the instance was open, some of the tabs will not have content in them.

To Reproduce

Steps to reproduce the behavior:

1. Open or be using VS Code.
2. Use the Command Pallet to bring up the Q# Help tab.
3. Use the Command Pallet to bring up the Q# API Documentation tab.
4. Make sure that only one of those tabs is visible (the Documentation tab for this example).
5. Close VS Code.
6. Reopen Vs Code.
7. Note that the Documentation tab is visible and filled in.
8. Use the Command Pallet to bring up the non-visible tab (the Help tab in this example).
9. It will bring up a new Help tab. (this is incorrect behavior)
10. After viewing the new Help tab, navigate to the old Help tab.
11. Notice that the old Help tab will be empty. (this is incorrect behavior)

Expected behavior

When you use the Command Pallet to bring up the Help tab after reopening VS Code, it should navigate you to the existing Help tab, rather than creating a new Help tab.