[jdk20] RFR: 8301989: new javax.swing.text.DefaultCaret().setBlinkRate(N) results in NPE

[Alexander Zuev]

Check if the component is associated with the caret before calling methods from it. Added test case that will make sure that will not happen again.

-------------

Commit messages:
 - 8301989: new javax.swing.text.DefaultCaret().setBlinkRate(N) results in NPE

Changes: https://git.openjdk.org/jdk20/pull/122/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=122&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301989
  Stats: 48 lines in 2 files changed: 46 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk20/pull/122.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/122/head:pull/122

PR: https://git.openjdk.org/jdk20/pull/122