Fix flow issue #6349

[archiecobbs]

Description

Commands enqueued by VaadinSession.access() in general have nothing to do with each other. The only thing they have in common is they share the same VaadinSession (and, by implication, VaadinService).

Therefore, if command №1 invoked UI.setCurrent() and command №2 invokes UI.getCurrent(), command №2 should read null, not the random UI from command №1 that it has nothing to do with.

Fixes #6349

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change
  • Except JarContentsManagerTest.copyFilesFromJar_doNotUpdateFileIfContentIsTheSame which has nothing to do with this change.
• I have performed self-review and corrected misspellings.