<Swing Dev> RFR: 5015261: JInternalFrame causes NPE

[Prasanta Sadhukhan]

JInternalFrame.getDesktopIcon() can be null as setDesktopIcon() can be called with null value. There are many places in JInternalFrame where getDesktopIcon() is accessed without null check which might cause NPE. 
Added null check for those cases.

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Commit messages:
 - 5015261: JInternalFrame causes NPE

Changes: https://git.openjdk.java.net/jdk/pull/4989/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4989&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-5015261
  Stats: 72 lines in 2 files changed: 63 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4989.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4989/head:pull/4989

PR: https://git.openjdk.java.net/jdk/pull/4989