Fix potential NPE (#320)

* Fix potential NPE

* Fix imports

spring-cloud-kubernetes-discovery/src/main/java/org/springframework/cloud/kubernetes/discovery/KubernetesDiscoveryClient.java

@@ -94,7 +94,7 @@ public List<ServiceInstance> getInstances(String serviceId) {
 				"[Assertion failed] - the object argument must be null");
 
 		Endpoints endpoints = client.endpoints().withName(serviceId).get();
-		List<EndpointSubset> subsets = null != endpoints ? endpoints.getSubsets() : new ArrayList<>();
+		List<EndpointSubset> subsets = getSubsetsFromEndpoints(endpoints);
 		List<ServiceInstance> instances = new ArrayList<>();
 		if (!subsets.isEmpty()) {
 
@@ -156,6 +156,17 @@ public List<ServiceInstance> getInstances(String serviceId) {
 		return instances;
 	}
 
+	private List<EndpointSubset> getSubsetsFromEndpoints(Endpoints endpoints) {
+		if (endpoints == null) {
+			return new ArrayList<>();
+		}
+		if (endpoints.getSubsets() == null) {
+			return new ArrayList<>();
+		}
+
+		return endpoints.getSubsets();
+	}
+
 	// returns a new map that contain all the entries of the original map
 	// but with the keys prefixed
 	// if the prefix is null or empty, the map itself is returned (unchanged of course)