Error when installing plugin: IllegalArgumentException: Invalid complete key specified: null @ /admin/plugins.vm

Problem description: When you install a plugin compiled with Java 5 in a Confluence application running with Java 1.4, you get the following error:

org.apache.velocity.exception.MethodInvocationException: Invocation of method 'isModuleEnabled' in class com.atlassian.confluence.admin.actions.plugins.ManagePluginsAction threw exception java.lang.IllegalArgumentException: Invalid complete key specified: null @ /admin/plugins.vm[109,42]
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:286)
caused by: java.lang.IllegalArgumentException: Invalid complete key specified: null
at com.atlassian.plugin.ModuleCompleteKey.(ModuleCompleteKey.java:17)

You cannot view the plugin in the plugin manager to uninstall it. See the workaround below.

Desired behaviour: Confluence should disable the plugin and show a meaningful error to the user.

Cause: This occurs when uploading a plugin into Confluence running an older version of Java than the plugin was compiled with.

Workaround: Run Confluence with the same version of Java as the plugin or, if you are the plugin author, recompile the plugin to Java 1.4 to support all Confluence instances. The first option is the way to uninstall a plugin with this problem in affected versions of Confluence.

Technical notes:

The causal exception is shown further up in the logs:

2008-06-23 10:40:31,005 ERROR [http-8080-Processor3] [atlassian.plugin.parsers.XmlDescriptorParser] createModuleDescriptor There was a problem loading the descriptor for module 'velocity-context-item' in plugin 'Metova Theme'.
Class version is incompatible with current JVM: com.metova.confluence.ThemeHelper

• url: /admin/uploadplugin.action | userName: admin | action: uploadplugin
  com.atlassian.plugin.PluginParseException: Class version is incompatible with current JVM: com.metova.confluence.ThemeHelper
  at com.atlassian.plugin.descriptors.AbstractModuleDescriptor.init(AbstractModuleDescriptor.java:75)
  at com.atlassian.confluence.plugin.descriptor.VelocityContextItemModuleDescriptor.init(VelocityContextItemModuleDescriptor.java:24)
  at com.atlassian.plugin.parsers.XmlDescriptorParser.createModuleDescriptor(XmlDescriptorParser.java:147)
  at com.atlassian.plugin.parsers.XmlDescriptorParser.configurePlugin(XmlDescriptorParser.java:85)
  at com.atlassian.plugin.loaders.ClassLoadingPluginLoader.deployPluginFromUnit(ClassLoadingPluginLoader.java:99)
  at com.atlassian.plugin.loaders.ClassLoadingPluginLoader.addFoundPlugins(ClassLoadingPluginLoader.java:151)
  at com.atlassian.confluence.plugin.DatabaseClassLoadingPluginLoader.addFoundPlugins(DatabaseClassLoadingPluginLoader.java:116)

This problem was also reported on the forum here:

http://forums.atlassian.com/thread.jspa?threadID=25904&tstart=-1