Produce a helpful error message if optionalProperty#field is bogus.

Otherwise get a useless “null is missing its descriptor” from Jenkins.getDescriptorOrDie.

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

@@ -31,7 +31,6 @@ import hudson.BulkChange;
 import hudson.Util;
 import hudson.model.listeners.SaveableListener;
 import hudson.util.FormApply;
-import hudson.util.QuotedStringTokenizer;
 import hudson.util.ReflectionUtils;
 import hudson.util.ReflectionUtils.Parameter;
 import hudson.views.ListViewColumn;
@@ -72,6 +71,8 @@ import java.lang.reflect.Type;
 import java.lang.reflect.Field;
 import java.lang.reflect.ParameterizedType;
 import java.beans.Introspector;
+import javax.annotation.CheckForNull;
+import javax.annotation.Nonnull;
 
 /**
  * Metadata about a configurable instance.
@@ -470,11 +471,27 @@ public abstract class Descriptor<T extends Describable<T>> implements Saveable {
     /**
      * Used by Jelly to abstract away the handlign of global.jelly vs config.jelly databinding difference.
      */
-    public PropertyType getPropertyType(Object instance, String field) {
+    public @CheckForNull PropertyType getPropertyType(@Nonnull Object instance, @Nonnull String field) {
         // in global.jelly, instance==descriptor
         return instance==this ? getGlobalPropertyType(field) : getPropertyType(field);
     }
 
+    /**
+     * Akin to {@link #getPropertyType(Object,String) but never returns null.
+     * @throws AssertionError in case the field cannot be found
+     * @since 1.492
+     */
+    public @Nonnull PropertyType getPropertyTypeOrDie(@Nonnull Object instance, @Nonnull String field) {
+        PropertyType propertyType = getPropertyType(instance, field);
+        if (propertyType != null) {
+            return propertyType;
+        } else if (instance == this) {
+            throw new AssertionError(getClass().getName() + " has no property " + field);
+        } else {
+            throw new AssertionError(clazz.getName() + " has no property " + field);
+        }
+    }
+
     /**
      * Obtains the property type of the given field of {@link #clazz}
      */

core/src/main/resources/lib/form/optionalProperty.jelly

@@ -22,7 +22,7 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 -->
 
-<?jelly escape-by-default='true'?>
+<?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">
   <st:documentation>
     Renders inline an optional single-value nested data-bound property of the current instance,
@@ -39,7 +39,7 @@ THE SOFTWARE.
     so override @checked manually.
   -->
   <f:optionalBlock field="${field}" title="${title}" checked="${instance[field]!=null}">
-    <j:set var="descriptor" value="${app.getDescriptorOrDie(descriptor.getPropertyType(instance,field).clazz)}" />
+    <j:set var="descriptor" value="${app.getDescriptorOrDie(descriptor.getPropertyTypeOrDie(instance,field).clazz)}" />
     <j:set var="instance" value="${instance[field]}"/>
     <st:include from="${descriptor}" page="${descriptor.configPage}" />
   </f:optionalBlock>

core/src/main/resources/lib/form/property.jelly

@@ -35,7 +35,7 @@ THE SOFTWARE.
     </st:attribute>
   </st:documentation>
   <f:rowSet name="${field}">
-    <j:set var="descriptor" value="${attrs.propertyDescriptor ?: app.getDescriptorOrDie(descriptor.getPropertyType(instance,field).clazz)}" />
+    <j:set var="descriptor" value="${attrs.propertyDescriptor ?: app.getDescriptorOrDie(descriptor.getPropertyTypeOrDie(instance,field).clazz)}" />
     <j:set var="instance" value="${instance[field]}"/>
     <st:include from="${descriptor}" page="${descriptor.configPage}" />
   </f:rowSet>