Index: trunk/src/org/openstreetmap/josm/tools/Utils.java
===================================================================
--- trunk/src/org/openstreetmap/josm/tools/Utils.java	(revision 19123)
+++ trunk/src/org/openstreetmap/josm/tools/Utils.java	(revision 19124)
@@ -666,6 +666,6 @@
             return Collections.singletonMap(entry.getKey(), entry.getValue());
         }
-        // Map.copyOf would also work, but if the original map is immutable, it just returns the original map.
-        return Map.ofEntries(map.entrySet().toArray(new Map.Entry[0]));
+        // see #23748 don't use Map.ofEntries as it doesn't allow null keys or values
+        return Collections.unmodifiableMap(map);
     }