[FIXED JENKINS-37997] FIX NPE when descriptor is not in DescriptorList (

jenkinsci#2536)

For example when Descriptor was defined as field and without @extension .

Signed-off-by: Kanstantsin Shautsou <kanstantsin.sha@gmail.com>

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

@@ -638,7 +638,13 @@ public Object instantiate(Class actualType, JSONObject json) {
             if (isApplicable(actualType, json)) {
                 LOGGER.log(Level.FINE, "switching to newInstance {0} {1}", new Object[] {actualType.getName(), json});
                 try {
-                    return Jenkins.getActiveInstance().getDescriptor(actualType).newInstance(Stapler.getCurrentRequest(), json);
+                    final Descriptor descriptor = Jenkins.getActiveInstance().getDescriptor(actualType);
+                    if (descriptor != null) {
+                        return descriptor.newInstance(Stapler.getCurrentRequest(), json);
+                    } else {
+                        LOGGER.log(Level.WARNING, "Descriptor not found. Falling back to default instantiation "
+                                + actualType.getName() + " " + json);
+                    }
                 } catch (Exception x) {
                     LOGGER.log(Level.WARNING, "falling back to default instantiation " + actualType.getName() + " " + json, x);
                     // If nested objects are not using newInstance, bindJSON will wind up throwing the same exception anyway,

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

@@ -1414,6 +1414,7 @@ public Descriptor getDescriptorByName(String id) {
      * If you have an instance of {@code type} and call {@link Describable#getDescriptor()},
      * you'll get the same instance that this method returns.
      */
+    @CheckForNull
     public Descriptor getDescriptor(Class<? extends Describable> type) {
         for( Descriptor d : getExtensionList(Descriptor.class) )
             if(d.clazz==type)