MapWithAILayerInfo: Fix CME

Signed-off-by: Taylor Smock <tsmock@fb.com>

src/main/java/org/openstreetmap/josm/plugins/mapwithai/data/mapwithai/MapWithAILayerInfo.java

@@ -423,6 +423,7 @@ public class MapWithAILayerInfo {
         boolean changed = false;
         Collection<String> knownDefaults = new TreeSet<>(Config.getPref().getList(CONFIG_PREFIX + "layers.default"));
         Collection<String> newKnownDefaults = new TreeSet<>();
+        synchronized (defaultLayers) {
             for (MapWithAIInfo def : defaultLayers) {
                 if (def.isDefaultEntry()) {
                     boolean isKnownDefault = false;
@@ -461,6 +462,7 @@ public class MapWithAILayerInfo {
                     }
                 }
             }
+        }
         if (!dropold && !knownDefaults.isEmpty()) {
             newKnownDefaults.addAll(knownDefaults);
         }