Ticket #17534: crash.patch

src/org/openstreetmap/josm/plugins/JunctionChecker/JunctionCheckerMapMode.java

@@ -41 +41 @@
         super.enterMode();
         MainApplication.getMap().mapView.addMouseListener(this);
         MainApplication.getMap().mapView.addMouseMotionListener(this);
-        MainApplication.getLayerManager().addActiveLayerChangeListener(this);
+        try {
+                MainApplication.getLayerManager().addActiveLayerChangeListener(this);
+        } catch (IllegalArgumentException e) {
+                // This will (hopefully) only occur when this has already been added...
+        }
     }
 
     @Override public void exitMode() {