Index: trunk/src/org/openstreetmap/josm/gui/dialogs/layer/DuplicateAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/layer/DuplicateAction.java	(revision 11504)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/layer/DuplicateAction.java	(revision 11505)
@@ -71,5 +71,5 @@
     public void updateEnabledState() {
         if (layer == null) {
-            if (model.getSelectedLayers().size() == 1) {
+            if (model != null && model.getSelectedLayers().size() == 1) {
                 setEnabled(model.getSelectedLayers().get(0) instanceof OsmDataLayer);
             } else {
Index: trunk/test/unit/org/openstreetmap/josm/gui/dialogs/layer/DuplicateActionTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/gui/dialogs/layer/DuplicateActionTest.java	(revision 11505)
+++ trunk/test/unit/org/openstreetmap/josm/gui/dialogs/layer/DuplicateActionTest.java	(revision 11505)
@@ -0,0 +1,58 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui.dialogs.layer;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+
+import java.io.InputStream;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.openstreetmap.josm.Main;
+import org.openstreetmap.josm.TestUtils;
+import org.openstreetmap.josm.gui.layer.OsmDataLayer;
+import org.openstreetmap.josm.io.OsmReader;
+import org.openstreetmap.josm.testutils.JOSMTestRules;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Unit tests of {@link DuplicateAction} class.
+ */
+public class DuplicateActionTest {
+    /**
+     * TMS layer needs prefs. Platform for LayerListDialog shortcuts.
+     */
+    @Rule
+    @SuppressFBWarnings(value = "URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD")
+    public JOSMTestRules test = new JOSMTestRules().preferences();
+
+    /**
+     * Non-regression test for ticket <a href="https://josm.openstreetmap.de/ticket/4539">#4539</a>.
+     * @throws Exception if an error occurs
+     */
+    @Test
+    public void testTicket4539() throws Exception {
+        try (InputStream is = TestUtils.getRegressionDataStream(4539, "josm_error_#4539.osm.zip")) {
+            OsmDataLayer layer = new OsmDataLayer(OsmReader.parseDataSet(is, null), null, null);
+            OsmDataLayer editLayer = Main.getLayerManager().getEditLayer();
+            assertNull(editLayer);
+            try {
+                new DuplicateAction(layer, null).actionPerformed(null);
+                editLayer = Main.getLayerManager().getEditLayer();
+                assertNotNull(editLayer);
+                assertFalse(layer.equals(editLayer));
+                assertEquals(layer.data.getNodes().size(), editLayer.data.getNodes().size());
+                assertEquals(layer.data.getWays().size(), editLayer.data.getWays().size());
+                assertEquals(layer.data.getRelations().size(), editLayer.data.getRelations().size());
+            } finally {
+                // Ensure we clean the place before leaving, even if test fails.
+                if (editLayer != null) {
+                    Main.getLayerManager().removeLayer(editLayer);
+                }
+            }
+        }
+    }
+}