Resolve #3455132 "Improve custom field"

Closes #3455132

src/SchemaDotOrgMappingManager.php

@@ -100,7 +100,22 @@ class SchemaDotOrgMappingManager implements SchemaDotOrgMappingManagerInterface
 
     // Apply custom properties defaults.
     if (isset($defaults['properties'])) {
-      foreach ($defaults['properties'] as $property_name => $property) {
+      foreach ($defaults['properties'] as $property_name => &$property) {
+        // Check custom properties/fields and set defaults values.
+        if (!$this->schemaTypeManager->hasProperty($schema_type, $property_name)) {
+          if (!is_array($property)) {
+            throw new \Exception(sprintf("Custom '%s' property/field is not defined.", $property_name));
+          }
+          $property += [
+            'type' => 'string',
+            'name' => strtolower($property_name),
+            'label' => $property_name,
+            'description' => '',
+            'unlimited' => FALSE,
+            'required' => FALSE,
+          ];
+        }
+
         if ($property === FALSE) {
           // Unset the name to not have the property added.
           $mapping_defaults['properties'][$property_name]['name'] = '';