Fix scrutinizer issue on Tree::$tree

It seems like null of type null is incompatible with the declared type array of property $tree

src/Tree.php

@@ -244,7 +244,7 @@ public function generateOrderFromDependencies()
         }
 
         //Reinit the tree for use the main system of tree generator.
-        $this->tree = null;
+        $this->tree = [];
     }
 
     /**