RFR: 8257: Stacktrace Percentage Column Tooltip throwing Exception

[Suchita Chaturvedi]

Open Thread page. Click on any thread. Stacktrace view gets populated with the results. Mouse hover on percentage column values. The tooltip doesnt get populated.

While debugging I found an exception in the console.

This PR is to fix the issue.

After the fix the tooltip is populating properly and there is no exception in console.

<img width="749" alt="8257_TooltipFixed" src="https://github.com/user-attachments/assets/b56329cc-653a-438f-9568-8f3010d4cc23">

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Commit messages:
 - 8257: Stacktrace Percentage Column Tooltip throwing Exception

Changes: https://git.openjdk.org/jmc/pull/589/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=589&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8257
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/589.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/589/head:pull/589

PR: https://git.openjdk.org/jmc/pull/589