RFR: 6286501: JTabbedPane throws NPE from its stateChanged listener in particular case

[Prasanta Sadhukhan]

JTabbedPane throws NPE from its stateChanged listener as it tries to access uninstantiated TabbedPane.
Fix is to use the tabPane passed to stateChangeListener so that correct tabPane is accessed.

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

Commit messages:
 - 6286501: JTabbedPane throws NPE from its stateChanged listener in particular case

Changes: https://git.openjdk.org/jdk/pull/10216/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10216&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6286501
  Stats: 77 lines in 2 files changed: 76 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10216.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10216/head:pull/10216

PR: https://git.openjdk.org/jdk/pull/10216