Fix null check for clusterStatsConfiguration

Author: oneonestar

gateway-ha/src/main/java/io/trino/gateway/ha/module/HaGatewayProviderModule.java

@@ -65,6 +65,8 @@
 import java.util.Map;
 
 import static io.airlift.jaxrs.JaxrsBinder.jaxrsBinder;
+import static io.trino.gateway.ha.config.ClusterStatsMonitorType.INFO_API;
+import static io.trino.gateway.ha.config.ClusterStatsMonitorType.NOOP;
 import static java.util.Objects.requireNonNull;
 
 public class HaGatewayProviderModule
@@ -223,9 +225,13 @@ public RoutingGroupSelector getRoutingGroupSelector(@ForRouter HttpClient httpCl
     public ClusterStatsMonitor getClusterStatsMonitor(@ForMonitor HttpClient httpClient)
     {
         ClusterStatsConfiguration clusterStatsConfig = configuration.getClusterStatsConfiguration();
-        if (configuration.getBackendState() == null) {
+        if (clusterStatsConfig == null) {
             return new ClusterStatsInfoApiMonitor(httpClient, configuration.getMonitor());
         }
+        if (!(clusterStatsConfig.getMonitorType() == INFO_API || clusterStatsConfig.getMonitorType() == NOOP)
+                && configuration.getBackendState() == null) {
+            throw new IllegalArgumentException("BackendStateConfiguration is required for monitor type: " + clusterStatsConfig.getMonitorType());
+        }
         return switch (clusterStatsConfig.getMonitorType()) {
             case INFO_API -> new ClusterStatsInfoApiMonitor(httpClient, configuration.getMonitor());
             case UI_API -> new ClusterStatsHttpMonitor(configuration.getBackendState());