log null blueprint in destroy city thread

src/engine/workthreads/DestroyCityThread.java

@@ -95,6 +95,12 @@ public class DestroyCityThread implements Runnable {
             if (cityBuilding == null)
                 continue;
 
+            // check null bluepritn and log error
+            if (cityBuilding.getBlueprint() == null){
+                Logger.error("Null Blueprint for building ID: " + cityBuilding.getObjectUUID());
+                continue;
+            }
+
             // Do nothing with the banestone.  It will be removed elsewhere
 
             if (cityBuilding.getBlueprint().getBuildingGroup().equals(mbEnums.BuildingGroup.BANESTONE))