[Packman] Package list will not update if registry is changed while the list is open and cause errors

If the user changes the registry in their manifest.json while the Package Manager window is already opened, Unity will resolve packages however the Package Manager will not update and continue to reference the old data, causing any attempted installations to fail and throwing errors to the console such as:

Cannot perform upm operation: Package [com.unity.2d.animation@2.2.0-preview.4] cannot be found [NotFound]
UnityEditor.EditorApplication:Internal_CallUpdateFunctions()

Steps to reproduce
- Create/open a new Unity project
- Edit the /packages/manifest.json file in the project structure, adding the following line before the last closing brace, saving changes:
, "registry": "https://artifactory.prd.cds.internal.unity3d.com/artifactory/api/npm/upm-candidates"
- Open the Package Manager window (Window > Package Manager)
- Enable preview packages (Advanced > Show preview packages)
- Select a preview package from the Candidates registry
- With this window open, return to /packages/manifest.json file and revert the changes made previously, saving your change
- Observe Unity will refresh its file system and resolve packages, but the Package Manager UI will not update, and errors will be thrown to the console