[9] Review Request: 6206439 JInternalFrame.setLayer(Integer layer) should throw NullPointerException when layer=null

[Sergey Bylokhov]

Hello.
Please review the trivial fix for jdk9.
The "@throws NullPointerException" was added to the specification of the 
method. CCC request will be sent after technical review.

Bug: https://bugs.openjdk.java.net/browse/JDK-6206439
Webrev can be found at: http://cr.openjdk.java.net/~serb/6206439/webrev.00

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Best regards, Sergey.