[jdk17u] RFR: 4819544: SwingSet2 JTable Demo throws NullPointerException

[Alex Kasko]

Clean backport of a SwingSet2 demo fix. Change it to demo code only.

Additional testing:

 - [x] checked manually with SwingSet2 that NPE does not happen

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Commit messages:
 - Backport 64d18d45ef885d5cdd893b7f56236d3f22b4b84c

Changes: https://git.openjdk.java.net/jdk17u/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=163&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-4819544
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/163/head:pull/163

PR: https://git.openjdk.java.net/jdk17u/pull/163