Fixed Undefined Key error in nested relationship

Author: kohlerdominik

src/BaseRelation.php

@@ -158,7 +158,7 @@ public function addEagerConstraints(array $models)
         // The first model in the array is always the parent, so add the scope constraints based on that model.
         // @link https://github.com/laravel/framework/pull/25240
         // @link https://github.com/lazychaser/laravel-nestedset/issues/351
-        optional($models[0])->applyNestedSetScope($this->query);
+        optional(reset($models))->applyNestedSetScope($this->query);
 
         $this->query->whereNested(function (Builder $inner) use ($models) {
             // We will use this query in order to apply constraints to the