Avoid NPE on informers (#1630)

I found out when working on a CloudBees proprietary feature that
`KubernetesCloud#readResolve` is not always called on deserialization.
I think this will not happen in OSS, but let's stay on the safe side and
initialize the field before using it.

Author: amuniz

Diff for: src/main/java/org/csanchez/jenkins/plugins/kubernetes/KubernetesCloud.java

@@ -172,7 +172,7 @@ public class KubernetesCloud extends Cloud implements PodTemplateGroup {
      * namespace -> informer
      * Use to watch pod events per namespace.
      */
-    private transient Map<String, SharedIndexInformer<Pod>> informers = new ConcurrentHashMap<>();
+    private transient volatile Map<String, SharedIndexInformer<Pod>> informers = new ConcurrentHashMap<>();
 
     @DataBoundConstructor
     public KubernetesCloud(String name) {
@@ -1306,9 +1306,6 @@ private Object readResolve() {
         if (containerCap != null && containerCap == 0) {
             containerCap = null;
         }
-        if (informers == null) {
-            informers = new ConcurrentHashMap<>();
-        }
         return this;
     }
 
@@ -1321,6 +1318,15 @@ public Cloud reconfigure(@NonNull StaplerRequest req, JSONObject form) throws De
     }
 
     public void registerPodInformer(KubernetesSlave node) {
+        // even having readResolve initializing informers is not enough, there are some special cases where XStream will
+        // not call it, so let us make sure it is initialized before using
+        if (informers == null) {
+            synchronized (this) {
+                if (informers == null) {
+                    informers = new ConcurrentHashMap<>();
+                }
+            }
+        }
         informers.computeIfAbsent(node.getNamespace(), (n) -> {
             KubernetesClient client;
             try {

Diff for: src/main/java/org/csanchez/jenkins/plugins/kubernetes/watch/PodStatusEventHandler.java

@@ -50,7 +50,9 @@ private String formatPodStatus(PodCondition c, String phase, StringBuilder sb) {
             // not interesting
             return "";
         }
-        String formatted = String.format("%n\tPod [%s][%s] %s", phase, c.getReason(), c.getMessage());
+        String message = c.getMessage();
+        String formatted =
+                String.format("%n\tPod [%s][%s] %s", phase, c.getReason(), message != null ? message : "No message");
         return sb.indexOf(formatted) == -1 ? formatted : "";
     }