fixes api-platform#5998

Author: soyuka

src/JsonSchema/SchemaFactory.php

@@ -217,20 +217,23 @@ private function buildPropertySchema(Schema $schema, string $definitionName, str
             }
 
             $subSchema = $this->buildSchema($className, $format, $parentType, null, $subSchema, $serializerContext + [self::FORCE_SUBSCHEMA => true], false);
-            if ($isCollection) {
-                $propertySchema['items']['$ref'] = $subSchema['$ref'];
-                unset($propertySchema['items']['type']);
-                break;
-            }
 
-            if ($type->isNullable()) {
-                $propertySchema['anyOf'] = [['$ref' => $subSchema['$ref']], ['type' => 'null']];
-            } else {
-                $propertySchema['$ref'] = $subSchema['$ref'];
-            }
+            if (isset($subSchema['$ref'])) {
+                if ($isCollection) {
+                    $propertySchema['items']['$ref'] = $subSchema['$ref'];
+                    unset($propertySchema['items']['type']);
+                    break;
+                }
 
-            unset($propertySchema['type']);
-            break;
+                if ($type->isNullable()) {
+                    $propertySchema['anyOf'] = [['$ref' => $subSchema['$ref']], ['type' => 'null']];
+                } else {
+                    $propertySchema['$ref'] = $subSchema['$ref'];
+                }
+
+                unset($propertySchema['type']);
+                break;
+            }
         }
 
         $schema->getDefinitions()[$definitionName]['properties'][$normalizedPropertyName] = new \ArrayObject($propertySchema);