Rails 7 系统测试由于模型回调而出现错误，我该如何解决这个问题？

问：

我正在尝试测试我的表单行为，但是当我单击保存时，回调会抛出如下所示的错误。 如果我评论下面的代码，一切正常。在这里，我不想测试回调，但它阻止了我成功测试行为。

我能做些什么来解决这个问题。

#before_create :assign_depth
#before_save   :assign_depth

fond.yml |夹具

first:
  name: At vero eos et accusamus
  explanation: some explanation
  parent: 
second:
  name: laborum et dolorum fuga. Et
  explanation: some explanation
  parent: first

fond.rb 模型

class Fond < ApplicationRecord
    ...
  before_create :assign_depth
  before_save   :assign_depth
  
  belongs_to :parent, class_name: "Fond", optional: true
  has_many :children, class_name: "Fond", foreign_key: "parent_id", dependent: :nullify

  private
    def assign_depth
      self.depth = (self.parent.present? ? parent.depth + 1 : 0)
    end
end

测试

require "application_system_test_case"

class FondsTest < ApplicationSystemTestCase
  setup do
    login_as users(:admin)
    @fond_related = fonds(:second)
  end

  test "should create related fond" do
    visit fonds_url
    click_on "New fond"

    fill_in "Name", with: "Test Test Test"
    fill_in "Explanation", with: @fond_related.explanation
    first(:xpath, "/html/body/div[2]/div[2]/form/div[3]/div").click()
    
    find('div.item', text: @fond_related.name).click()

    click_on ("Fond Kaydet")

    assert_text "Fond was successfully created"
  end
end

测试结果

E

Error:
FondsTest#test_should_create_related_fond:
NoMethodError: undefined method `+' for nil:NilClass
    app/models/fond.rb:29:in `assign_depth'
    app/controllers/fonds_controller.rb:29:in `block in create'
    app/controllers/fonds_controller.rb:28:in `create'