Last window closed should be immutable

[bbondy]

Auditors: @bsclifton

Fix #10423

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

LGTM 😄 👍 I checked for other instances of incorrect session usage and didn't find any. A++, best PR ever! 🥇

[codecov-io]

Codecov Report

Merging #10437 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10437      +/-   ##
==========================================
- Coverage   53.81%    53.8%   -0.01%     
==========================================
  Files         238      238              
  Lines       21051    21051              
  Branches     3258     3258              
==========================================
- Hits        11329    11327       -2     
- Misses       9722     9724       +2

Flag	Coverage Δ
#unittest	53.8% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
app/sessionStoreShutdown.js	85.8% <100%> (ø)	⬆️
app/sessionStore.js	65.11% <0%> (-0.35%)	⬇️

[bbondy]

Merged 0.18.x, 0.19.x, 0.20.x, master.