Fix LT-22633 match case box checked

[AndyBlack]

This is similar to the fix in #1025. I forgot to test the case where just the "March case" checkbox is checked while looking at the Problems view: since the view is no longer on the Grammar Sketch, the web bowser's window is null.

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   10m 48s ⏱️ -3s
4 313 tests ±0  4 240 ✅ ±0  73 💤 ±0  0 ❌ ±0 
4 322 runs  ±0  4 249 ✅ ±0  73 💤 ±0  0 ❌ ±0 

Results for commit 14b52c2. ± Comparison against base commit 330c87d.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.04%. Comparing base (330c87d) to head (14b52c2).

Files with missing lines	Patch %	Lines
Src/xWorks/GeneratedHtmlViewer.cs	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1030      +/-   ##
==========================================
+ Coverage   33.02%   33.04%   +0.01%     
==========================================
  Files        1202     1202              
  Lines      278236   278240       +4     
  Branches    37170    37171       +1     
==========================================
+ Hits        91888    91931      +43     
+ Misses     158498   158496       -2     
+ Partials    27850    27813      -37     

Files with missing lines	Coverage Δ
Src/xWorks/GeneratedHtmlViewer.cs	0.00% <0.00%> (ø)

... and 7 files with indirect coverage changes

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