Add ExtensionList.lookupFirst convenience method.

core/src/main/java/hudson/ExtensionList.java

@@ -459,6 +459,31 @@ public static <T> ExtensionList<T> create(Jenkins jenkins, Class<T> type) {
         return all.get(0);
     }
 
+    /**
+     * Convenience method allowing lookup of the instance of a given type with the highest ordinal.
+     * Equivalent to {@code ExtensionList.lookup(type).get(0)} if there is at least one instance,
+     * and throws an {@link IllegalStateException} otherwise if no instance could be found.
+     *
+     * @param type The type to look up.
+     * @return the singleton instance of the given type in its list.
+     * @throws IllegalStateException if there are no instances
+     *
+     * @since TODO
+     */
+    public static @NonNull <U> U lookupFirst(Class<U> type) {
+        var all = lookup(type);
+        if (!all.isEmpty()) {
+            return all.get(0);
+        } else {
+            if (Main.isUnitTest) {
+                throw new IllegalStateException("Found no instances of " + type.getName() +
+                        " registered (possible annotation processor issue); try using `mvn clean test -Dtest=…` rather than an IDE test runner");
+            } else {
+                throw new IllegalStateException("Found no instances of " + type.getName() + " registered");
+            }
+        }
+    }
+
     /**
      * Places to store static-scope legacy instances.
      */

test/src/test/java/lib/form/OptionTest.java

@@ -28,6 +28,7 @@
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertTrue;
 
+import hudson.ExtensionList;
 import hudson.model.RootAction;
 import org.htmlunit.html.DomElement;
 import org.htmlunit.html.DomNodeList;
@@ -216,7 +217,7 @@ private void checkJelly(int mode, String msgToInject,
                             String bodyContainsExpected, String valueContainsExpected,
                             boolean checkExactCharacters,
                             boolean withValueTrue, boolean withValueFalse) throws Exception {
-        UsingJellyView view = j.jenkins.getExtensionList(UsingJellyView.class).get(0);
+        UsingJellyView view = ExtensionList.lookupFirst(UsingJellyView.class);
         view.setMode(mode);
         view.setInjection(msgToInject);
 
@@ -242,7 +243,7 @@ private void checkGroovy(int mode, String msgToInject,
                              String bodyContainsExpected, String valueContainsExpected,
                              boolean checkExactCharacters,
                              boolean withValueTrue, boolean withValueFalse) throws Exception {
-        UsingGroovyView view = j.jenkins.getExtensionList(UsingGroovyView.class).get(0);
+        UsingGroovyView view = ExtensionList.lookupFirst(UsingGroovyView.class);
         view.setMode(mode);
         view.setInjection(msgToInject);