Index: applications/editors/josm/plugins/native-password-manager/src/org/openstreetmap/josm/plugins/npm/NPMPlugin.java
===================================================================
--- applications/editors/josm/plugins/native-password-manager/src/org/openstreetmap/josm/plugins/npm/NPMPlugin.java	(revision 31603)
+++ applications/editors/josm/plugins/native-password-manager/src/org/openstreetmap/josm/plugins/npm/NPMPlugin.java	(revision 31665)
@@ -1,4 +1,6 @@
 // License: GPL. For details, see LICENSE file.
 package org.openstreetmap.josm.plugins.npm;
+
+import java.awt.GraphicsEnvironment;
 
 import javax.swing.SwingUtilities;
@@ -26,5 +28,5 @@
                     new NPMCredentialsAgentFactory(sel)
             );
-        } else {
+        } else if (!GraphicsEnvironment.isHeadless()) {
             SwingUtilities.invokeLater(new Runnable() {
                 @Override