[JENKINS-34914] - Prevent NPE in Jenkins#getRootURL() when the nstance is not fully loaded

(cherry picked from commit c064d88a)

core/src/main/java/jenkins/model/Jenkins.java

@@ -2326,12 +2326,21 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
      * It is done in this order so that it can work correctly even in the face
      * of a reverse proxy.
      *
-     * @return null if this parameter is not configured by the user and the calling thread is not in an HTTP request; otherwise the returned URL will always have the trailing {@code /}
+     * @return {@code null} if this parameter is not configured by the user and the calling thread is not in an HTTP request; 
+     *                      otherwise the returned URL will always have the trailing {@code /}
+     * @throws IllegalStateException {@link JenkinsLocationConfiguration} cannot be retrieved.
+     *                      Jenkins instance may be not ready, or there is an extension loading glitch.
      * @since 1.66
      * @see <a href="https://wiki.jenkins-ci.org/display/JENKINS/Hyperlinks+in+HTML">Hyperlinks in HTML</a>
      */
-    public @Nullable String getRootUrl() {
-        String url = JenkinsLocationConfiguration.get().getUrl();
+    public @Nullable String getRootUrl() throws IllegalStateException {
+        final JenkinsLocationConfiguration config = JenkinsLocationConfiguration.get();
+        if (config == null) {
+            // Try to get standard message if possible
+            final Jenkins j = Jenkins.getInstance();
+            throw new IllegalStateException("Jenkins instance " + j + " has been successfully initialized, but JenkinsLocationConfiguration is undefined.");
+        }
+        String url = config.getUrl();
         if(url!=null) {
             return Util.ensureEndsWith(url,"/");
         }

core/src/main/java/jenkins/model/JenkinsLocationConfiguration.java

@@ -41,6 +41,12 @@ public class JenkinsLocationConfiguration extends GlobalConfiguration {
     // just to suppress warnings
     private transient String charset,useSsl;
 
+    /**
+     * Gets local configuration.
+     * 
+     * @return {@code null} if the {@link GlobalConfiguration#all()} list does not contain this extension.
+     *         Most likely it means that the Jenkins instance has not been fully loaded yet.
+     */
     public static @CheckForNull JenkinsLocationConfiguration get() {
         return GlobalConfiguration.all().get(JenkinsLocationConfiguration.class);
     }