Make isRoot() call on annotation safe

src/Analysis.php

@@ -338,7 +338,7 @@ public function getSchemaForSource(string $fqdn): ?OA\Schema
                 $definition = $definitions[$fqdn];
                 if (is_iterable($definition['context']->annotations)) {
                     foreach (array_reverse($definition['context']->annotations) as $annotation) {
-                        if ($annotation->isRoot(OA\Schema::class) && !$annotation->_context->is('generated')) {
+                        if ($annotation instanceof OA\AbstractAnnotation && $annotation->isRoot(OA\Schema::class) && !$annotation->_context->is('generated')) {
                             return $annotation;
                         }
                     }

tests/Fixtures/Scratch/ThirdPartyAnnotation.php

@@ -0,0 +1,41 @@
+<?php declare(strict_types=1);
+
+// intentionally none: namespace OpenApi\Tests\Fixtures\Scratch;
+
+use OpenApi\Annotations as OA;
+
+/**
+ * @Annotation
+ */
+class ThirdPartyAnnotation
+{
+}
+
+/**
+ * @OA\Schema
+ * @ThirdPartyAnnotation
+ */
+class SomeParent
+{
+}
+
+/**
+ * @OA\Schema
+ */
+class Child extends SomeParent
+{
+}
+
+/**
+ * @OA\Info(title="API", version="1.0")
+ * @OA\Get(
+ *     path="/api/endpoint",
+ *     @OA\Response(
+ *         response=200,
+ *         description="successful operation"
+ *     )
+ * )
+ */
+class ThirdPartyAnnotationEndpoint
+{
+}

tests/Fixtures/Scratch/ThirdPartyAnnotation.yaml

@@ -0,0 +1,17 @@
+openapi: 3.0.0
+info:
+  title: API
+  version: '1.0'
+paths:
+  /api/endpoint:
+    get:
+      responses:
+        '200':
+          description: 'successful operation'
+components:
+  schemas:
+    SomeParent: {  }
+    Child:
+      allOf:
+        -
+          $ref: '#/components/schemas/SomeParent'