Index: src/org/openstreetmap/josm/actions/ExtensionFileFilter.java
===================================================================
--- src/org/openstreetmap/josm/actions/ExtensionFileFilter.java	(Revision 1975)
+++ src/org/openstreetmap/josm/actions/ExtensionFileFilter.java	(Arbeitskopie)
@@ -42,6 +42,11 @@
      *
      */
     public ExtensionFileFilter(String extension, String defExt, String description) {
+        if (extension == null ||
+            defExt == null ||
+            description == null) {
+            throw new NullPointerException();
+        }
         this.extension = extension;
         defaultExtension = defExt;
         this.description = description;