JDK tested: jdk7-b55
Bundles location: http://jre.sfbay/java/re/jdk/7/nightly/bundles/windows-i586/latest
OS tested: Windows Vista, winXP Pro
Browser: IE/FF

The problem is reproducible on windows with FF only.

1. Install jre from the location above.
2. Use FF3 to load SwingSet2 Applet or Java 2D from http://java.sun.com/products/plugin/1.5.0/demos/applets.html
3. The exception is printed to the Java Console when the applet is closed.
This is the stack trace of the exception:

Java Plug-in 1.7.0-ea
Using JRE version 1.7.0-ea Java HotSpot(TM) Client VM
User home directory = C:\Users\admin


----------------------------------------------------
c: clear console window
f: finalize objects on finalization queue
g: garbage collect
h: display this help message
l: dump classloader list
m: print memory usage
o: trigger logging
p: reload proxy configuration
q: hide console
r: reload policy configuration
s: dump system and deployment properties
t: dump thread list
v: dump thread stack
x: clear classloader cache
0-5: set trace level to <n>
----------------------------------------------------

java.lang.NullPointerException
at sun.awt.windows.WEmbeddedFrame.synthesizeWindowActivation(Unknown Source)
at sun.plugin.viewer.WNetscapePluginObject.setFocus(Unknown Source)
at sun.plugin.services.WPlatformService.waitEvent(Native Method)
at sun.plugin.viewer.frame.WNetscapeEmbeddedFrame.waitEvent(Unknown Source)
at sun.plugin.viewer.frame.WNetscapeEmbeddedFrame.destroy(Unknown Source)
at sun.plugin.viewer.WNetscapePluginObject.destroyPlugin(Unknown Source)
at sun.plugin.viewer.WNetscapePluginObject.destroyPlugin(Unknown Source)

Could be a side effect of fixed CR 6799345