Index: applications/editors/josm/plugins/FastDraw/test/unit/org/openstreetmap/josm/plugins/fastdraw/DrawnPolyLineTest.java
===================================================================
--- applications/editors/josm/plugins/FastDraw/test/unit/org/openstreetmap/josm/plugins/fastdraw/DrawnPolyLineTest.java	(revision 36056)
+++ applications/editors/josm/plugins/FastDraw/test/unit/org/openstreetmap/josm/plugins/fastdraw/DrawnPolyLineTest.java	(revision 36057)
@@ -1,2 +1,3 @@
+// License: GPL. For details, see LICENSE file.
 package org.openstreetmap.josm.plugins.fastdraw;
 
Index: applications/editors/josm/plugins/FastDraw/test/unit/org/openstreetmap/josm/plugins/fastdraw/FastDrawingModeTest.java
===================================================================
--- applications/editors/josm/plugins/FastDraw/test/unit/org/openstreetmap/josm/plugins/fastdraw/FastDrawingModeTest.java	(revision 36057)
+++ applications/editors/josm/plugins/FastDraw/test/unit/org/openstreetmap/josm/plugins/fastdraw/FastDrawingModeTest.java	(revision 36057)
@@ -0,0 +1,39 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.plugins.fastdraw;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+import org.openstreetmap.josm.data.osm.DataSet;
+import org.openstreetmap.josm.gui.MainApplication;
+import org.openstreetmap.josm.gui.layer.OsmDataLayer;
+import org.openstreetmap.josm.testutils.annotations.Main;
+import org.openstreetmap.josm.testutils.annotations.Projection;
+
+/**
+ * Test class for {@link FastDrawingMode}
+ */
+@Main
+@Projection
+class FastDrawingModeTest {
+    /**
+     * Non-regression test for #21659: IAE: Listener {@link FastDrawingMode} was not registered before or already removed
+     */
+    @Test
+    void testNonRegression21659() {
+        final OsmDataLayer osmDataLayer = new OsmDataLayer(new DataSet(), "testNonRegression21659", null);
+        final FastDrawingMode mode = new FastDrawingMode();
+        assertFalse(mode.isEnabled());
+        MainApplication.getLayerManager().addLayer(osmDataLayer);
+        mode.updateEnabledState();
+        assertTrue(mode.isEnabled());
+        osmDataLayer.lock();
+        mode.updateEnabledState();
+        assertFalse(mode.isEnabled());
+        osmDataLayer.unlock();
+        mode.enterMode();
+        assertDoesNotThrow(mode::exitMode);
+    }
+}