add set root ids

backend/tests/Fakes/FakeElementRepository.php

@@ -37,6 +37,20 @@ class FakeElementRepository implements ElementRepository
         return $this->cloneElement($this->elementsById[$id]);
     }
 
+    public function findRootBySet(DomainSet $set): ?Element
+    {
+        foreach ($this->elementsById as $element) {
+            if (
+                $element->getSet()->getId() === $set->getId()
+                && $element->getParentElement() === null
+            ) {
+                return $this->cloneElement($element);
+            }
+        }
+
+        return null;
+    }
+
     /**
      * @return Element[]
      */