Do not block rendering of Manage Jenkins while waiting for UC data (#4881)

Co-authored-by: NDaniel Beck <daniel-beck@users.noreply.github.com>

core/src/main/java/hudson/PluginWrapper.java

@@ -1306,10 +1306,16 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
     public List<UpdateSite.Deprecation> getDeprecations() {
         /* Would be much nicer to go through getInfoFromAllSites but that only works for currently published plugins */
         List<UpdateSite.Deprecation> deprecations = new ArrayList<>();
-        for (UpdateSite site : Jenkins.get().getUpdateCenter().getSites()) {
-            for (Map.Entry<String, UpdateSite.Deprecation> entry : site.getData().getDeprecations().entrySet()) {
-                if (entry.getKey().equals(this.shortName)) {
-                    deprecations.add(entry.getValue());
+        final UpdateCenter updateCenter = Jenkins.get().getUpdateCenter();
+        if (updateCenter.isSiteDataReady()) {
+            for (UpdateSite site : updateCenter.getSites()) {
+                final UpdateSite.Data data = site.getData();
+                if (data != null) {
+                    for (Map.Entry<String, UpdateSite.Deprecation> entry : data.getDeprecations().entrySet()) {
+                        if (entry.getKey().equals(this.shortName)) {
+                            deprecations.add(entry.getValue());
+                        }
+                    }
                 }
             }
         }

core/src/main/java/hudson/model/UpdateSite.java

@@ -336,6 +336,7 @@ public class UpdateSite {
      *
      * @return  null if no data is available.
      */
+    @CheckForNull
     public Data getData() {
         if (data == null) {
             JSONObject o = getJSONObject();

core/src/main/java/jenkins/management/PluginsLink.java

@@ -26,11 +26,14 @@ package jenkins.management;
 
 import hudson.Extension;
 import hudson.model.ManagementLink;
+import hudson.model.UpdateCenter;
 import hudson.security.Permission;
 import jenkins.model.Jenkins;
 import org.jenkinsci.Symbol;
 
 import edu.umd.cs.findbugs.annotations.NonNull;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
 
 /**
  * @author <a href="mailto:nicolas.deloof@gmail.com">Nicolas De Loof</a>
@@ -68,4 +71,14 @@ public class PluginsLink extends ManagementLink {
     public Category getCategory() {
         return Category.CONFIGURATION;
     }
+
+    @Restricted(NoExternalUse.class)
+    public boolean hasUpdates() {
+        final UpdateCenter updateCenter = Jenkins.get().getUpdateCenter();
+        if (!updateCenter.isSiteDataReady()) {
+            // Do not display message during this page load, but possibly later.
+            return false;
+        }
+        return !updateCenter.getUpdates().isEmpty();
+    }
 }

core/src/main/resources/jenkins/management/PluginsLink/info.jelly

@@ -24,7 +24,7 @@
 
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler">
-  <j:if test="${!empty app.updateCenter.updates}">
+  <j:if test="${it.hasUpdates()}">
     <i class="fa fa-bell" aria-hidden="true" style="color: red"></i> ${%updates available}
   </j:if>
 </j:jelly>
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