How to save foreign key with null value?

I have 2 models, which are tree and branch.

The relationship between these model is 1 optional tree to many optional branch.

There is no issue to have 1 tree with 0 or many branch, but encounter issue when to have 1 branch with 0 tree.

Output: Message: The Tree does not exist. Field: tree_id Type: ConstraintViolation

<?php

//CREATE TABLE tree
//( tree_id VARCHAR NOT NULL DEFAULT md5(now()::TEXT||random()) PRIMARY KEY,
//  name VARCHAR NOT NULL
//);

class Tree extends Phalcon\Mvc\Model {
    public $tree_id; //VARCHAR NOT NULL DEFAULT md5(now()::TEXT||random()) PRIMARY KEY,
    public $name; //name NOT NULL,

    public function initialize() {
        $this->setSource('tree');

        $this->hasMany('tree_id', 'Branch', 'tree_id', array(
            'foreignKey' => array('message' => 'The Tree cannot be deleted because it is being used by Branch.')
        ));

    }
}

<?php

//CREATE TABLE branch
//( branch_id VARCHAR NOT NULL DEFAULT md5(now()::TEXT||random()) PRIMARY KEY,
//  tree_id VARCHAR DEFAULT NULL REFERENCES tree (tree_id),
//  name VARCHAR NOT NULL
//);

class Branch extends Phalcon\Mvc\Model {
    public $branch_id; //VARCHAR NOT NULL DEFAULT md5(now()::TEXT||random()) PRIMARY KEY,
    public $tree_id; //VARCHAR DEFAULT NULL REFERENCES tree (tree_id),
    public $name; //VARCHAR NOT NULL

    public function initialize() {
        $this->setSource('branch');

        $this->belongsTo('tree_id', 'Tree', 'tree_id', array(
            'foreignKey' => array('message' => 'The Tree does not exist.')
        ));
    }
}

<?php

class TestController extends Phalcon\Mvc\Controller {
    public function createAction() {

        \Phalcon\Mvc\Model::setup(array(
            'notNullValidations' => false
        ));

        $branch = new Branch();
        $branch->branch_id = 'branch1';
        $branch->name = 'branch 1';
        $branch->tree = null;
        $branch->tree_id = null;

        if (!$branch->save())
            foreach ($branch->getMessages() as $message) {
                echo "Message: ", $message->getMessage() . '<br/>';
                echo "Field: ", $message->getField() . '<br/>';
                echo "Type: ", $message->getType() . '<br/>';
            }
    }
}