Check for null IP address before notifying of an external IP update.

src/main/java/org/qortal/network/Network.java

@@ -1164,6 +1164,7 @@ public class Network {
         if (consecutiveReadings >= consecutiveReadingsRequired) {
             // Last 10 readings were the same - i.e. more than one peer agreed on the new IP address...
             String ip = ipAddressHistory.get(size - 1);
+            if (ip != null && !Objects.equals(ip, "null")) {
                 if (!Objects.equals(ip, this.ourExternalIpAddress)) {
                     // ... and the readings were different to our current recorded value, so
                     // update our external IP address value
@@ -1172,6 +1173,7 @@ public class Network {
                 }
             }
         }
+    }
 
     public void onExternalIpUpdate(String ipAddress) {
         LOGGER.info("External IP address updated to {}", ipAddress);