Index: src/org/openstreetmap/josm/plugins/PluginSelection.java
===================================================================
--- src/org/openstreetmap/josm/plugins/PluginSelection.java (revision 1738)
+++ src/org/openstreetmap/josm/plugins/PluginSelection.java (working copy)
@@ -94,7 +94,7 @@
 
     public boolean finish() {
         Collection<PluginInformation> toDownload = new LinkedList<PluginInformation>();
-        Collection<String> installedPlugins = Main.pref.getCollection("plugins", null);
+        Collection<String> installedPlugins = Main.pref.getCollection("plugins", Collections.<String>emptySet());
 
         String msg = "";
         for (Entry<String, Boolean> entry : pluginMap.entrySet()) {