Added debug code

src/engine/objects/Building.java

@@ -597,6 +597,11 @@ public class Building extends AbstractWorldObject {
             return;
         }
 
+        if (city.destroyed.get() == true) {
+            Logger.error(city.getName() + "attempt to destroy a destroyed city");
+            return;
+        }
+
         // Must remove a bane before considering destruction of a TOL
 
         if (bane != null) {