please review fix for 117384

Please review a fix for https://netbeans.org/bugzilla/show_bug.cgi?id=117384

NPE at
org.netbeans.modules.options.advanced.AdvancedPanel.update

Description:. In some random cases NullPointerException is thrown by
AdvancedPanel.update method when

Tools > Options > Miscellaneous is opened.

Impact: Miscellaneous panel stays empty. Looks like happens mostly in
stressful conditions (during building big projects etc.)

The problem was reported 117 times starting from NetBeans 6.0 Beta.

Risk: minimal. Race condition possibility is removed by synchronizing
getAdvancePanel() method.

Changeset: http://hg.netbeans.org/main/rev/6c3fbacf72d3

QA has tested the fix.

Thanks

Yulia