Index: /trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java	(revision 16105)
+++ /trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java	(revision 16106)
@@ -42,4 +42,5 @@
 
 import javax.imageio.ImageIO;
+import javax.swing.ListSelectionModel;
 import javax.xml.namespace.QName;
 import javax.xml.stream.XMLStreamException;
@@ -328,9 +329,17 @@
         }
 
+        @Override
+        public void setupDialog() {
+            super.setupDialog();
+            buttons.get(0).setEnabled(false);
+            ListSelectionModel selectionModel = list.getTable().getSelectionModel();
+            selectionModel.addListSelectionListener(e -> buttons.get(0).setEnabled(!selectionModel.isSelectionEmpty()));
+        }
+
         public DefaultLayer getSelectedLayer() {
             Layer selectedLayer = list.getSelectedLayer();
-            return new DefaultLayer(ImageryType.WMTS, selectedLayer.identifier, selectedLayer.style, selectedLayer.tileMatrixSet.identifier);
-        }
-
+            return selectedLayer == null ? null :
+                    new DefaultLayer(ImageryType.WMTS, selectedLayer.identifier, selectedLayer.style, selectedLayer.tileMatrixSet.identifier);
+        }
     }