<Swing Dev> RFR: 5078989: Null Pointer exception in SpinnerListMode

[Prasanta Sadhukhan]

Please review a fix for an issue where it is seen that when the data in SpinnerListModel has a null value, pressing any
key after deleting the current value throws null pointer exception. This is because findNextMatch() calls list.get but
dont check for null value and tries to call toString() on the value resulting in NPE. The spec is not clear about
SpinnerListModel having null values in List so user can have null values in List passed to SpinnerListModel constructor
so we need to guard against it for NPE.

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Commit messages:
 - Fix indent issue
 - 5078989: Null Pointer exception in SpinnerListMode

Changes: https://git.openjdk.java.net/jdk/pull/423/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=423&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-5078989
  Stats: 93 lines in 2 files changed: 90 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/423.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/423/head:pull/423

PR: https://git.openjdk.java.net/jdk/pull/423