<Swing Dev> RFR JDK-6709913: BasicComboBoxUI.isPopupVisible returns NullPointerException

[Prasanta Sadhukhan]

Hi All,

Please review a fix for an issue whereby 
javax.swing.plaf.basic.BasicComboBoxUI.isPopupVisible returns 
NullPointerException when called from a overridden method of getModel().

If a class extends JComboBox and overrides getModel() and then it calls 
isPopupVisible() from the overriden getModel() it gets 
NullPointerException the first time.

Because the popup variable in javax.swing.plaf.basic.BasicComboBoxUI is 
not set yet.

Proposed fix is to guard against it by doing a null check for the 
methods which can be overriden from JComboBox to access "popup" variable.

Bug: https://bugs.openjdk.java.net/browse/JDK-6709913

webrev: http://cr.openjdk.java.net/~psadhukhan/6709913/webrev.0/

Regards
Prasanta