erro log fix and zerg mechanic manager

src/engine/net/client/handlers/ManageCityAssetMsgHandler.java

@@ -294,7 +294,7 @@ public class ManageCityAssetMsgHandler extends AbstractClientMsgHandler {
 
         // Owner is obviously allowed to upgrade his own buildings
 
-        if (building.getOwner().equals(player)) {
+        if (building.getOwner() != null && building.getOwner().equals(player)) {
 
             // Players cannot destroy or transfer a TOL.