RFR: 8259: Several JFR pages throwing exceptions after JMC-8244

[Suchita Chaturvedi]

This PR is to fix the NPE issue on different screens of JMC after 8244 fix.
JVM Internals
Garbage Collections
GC Configuration
Class Loading
Recording

All these screens are working fine now.

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Commit messages:
 - 8259: Several JFR pages throwing exceptions after JMC-8244

Changes: https://git.openjdk.org/jmc/pull/590/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=590&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8259
  Stats: 94 lines in 1 file changed: 39 ins; 35 del; 20 mod
  Patch: https://git.openjdk.org/jmc/pull/590.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/590/head:pull/590

PR: https://git.openjdk.org/jmc/pull/590