Index: trunk/src/org/openstreetmap/josm/gui/dialogs/layer/ActivateLayerAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/layer/ActivateLayerAction.java	(revision 16789)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/layer/ActivateLayerAction.java	(revision 16790)
@@ -71,5 +71,10 @@
             toActivate = layer;
         } else {
-            toActivate = model.getSelectedLayers().get(0);
+            List<Layer> layers = model.getSelectedLayers();
+            if (layers.isEmpty()) {
+                // see #19476 for a possible cause
+                return;
+            }
+            toActivate = layers.get(0);
         }
         execute(toActivate);