Fix LT-22581: Crash clicking Values expansion button

[jtmaxwell3]

This fixes https://jira.sil.org/browse/LT-22581. The code crashed because m_key wasn't initialized.

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   10m 47s ⏱️ +13s
4 294 tests ±0  4 221 ✅ ±0  73 💤 ±0  0 ❌ ±0 
4 303 runs  ±0  4 230 ✅ ±0  73 💤 ±0  0 ❌ ±0 

Results for commit 52e9bb1. ± Comparison against base commit cf8d588.

[jasonleenaylor]

@jasonleenaylor made 1 comment.
Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on jtmaxwell3).

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Src/Common/Controls/DetailControls/DataTree.cs line 5305 at r1 (raw file):

			m_node = node;
			m_path = path;
			m_key = path.ToArray(); // This fixes LT-22581.

This seemed like too big of an oversight to have lived this long so I investigated a bit. There are multiple logic gates in the code which use a null key value as a sentinel. This will have behavior changes beyond the crash. I think a guard in the crash location might be better. You could even ask Claude to look for any other places where the key needs a guard.