Class sun.applet.PluginAppletViewer removes the internal class sun.misc.Ref, which has been removed from OpenJDK 9. The recommended replacement is java.lang.ref.SoftReference, see <https://bugs.openjdk.java.net/browse/JDK-8032397> and <http://mail.openjdk.java.net/pipermail/core-libs-dev/2014-January/024559.html>.
fyi - http://mail.openjdk.java.net/pipermail/distro-pkg-dev/2014-October/029842.html