IcedTea6 1.12.6 browser plugin crashes when loading pages with Java applets

Here's what's shown when calling the browser from a shell:

  java version "1.6.0_27"
  OpenJDK Runtime Environment (IcedTea6 1.12.6) (6b27-1.12.6-1ubuntu0.10.04.2)
  OpenJDK Server VM (build 20.0-b12, mixed mode)
  java.lang.NullPointerException
   at javax.swing.SwingUtilities.appContextGet(SwingUtilities.java:1858)
   at javax.swing.SwingUtilities.getSharedOwnerFrame(SwingUtilities.java:1828)
   at javax.swing.JWindow.<init>(JWindow.java:185)
   at javax.swing.JWindow.<init>(JWindow.java:137)
   at net.sourceforge.jnlp.runtime.JNLPSecurityManager.<init>(JNLPSecurityManager.java:121)
   at net.sourceforge.jnlp.runtime.JNLPRuntime.initialize(JNLPRuntime.java:238)
   at sun.applet.PluginAppletSecurityContext.<init>(PluginAppletSecurityContext.java:245)
   at sun.applet.PluginMain.main(PluginMain.java:109)
  Something very bad happened. I don't know what to do, so I am going to exit :(

I detected the issue while running Seamonkey 2.20 on web pages containing embedded Java applets (here's a handy one to test: http://www.bitstorm.org/gameoflife/), but I know it also happens with previous versions of Seamonkey, and I suspect it also happens with other browsers.

The workaround is to downgrade the IcedTea6 packages to the immediately preceding version:

  cd /tmp
  wget https://launchpad.net/~ubuntu-security/+archive/ppa/+build/4519908/+files/openjdk-6-jre-headless_6b27-1.12.5-0ubuntu0.10.04.1_i386.deb \
   https://launchpad.net/~ubuntu-security/+archive/ppa/+build/4519908/+files/icedtea-6-jre-cacao_6b27-1.12.5-0ubuntu0.10.04.1_i386.deb \
   https://launchpad.net/~ubuntu-security/+archive/ppa/+build/4519908/+files/openjdk-6-jre_6b27-1.12.5-0ubuntu0.10.04.1_i386.deb
  sudo dpkg -i openjdk-6-jre-headless_6b27-1.12.5-0ubuntu0.10.04.1_i386.deb \
   icedtea-6-jre-cacao_6b27-1.12.5-0ubuntu0.10.04.1_i386.deb \
   openjdk-6-jre_6b27-1.12.5-0ubuntu0.10.04.1_i386.deb