TakeAssementController中的Nomethod错误#start

Question

  

nil的未定义方法'id'：NilClass围绕行提取源   42

这是我的take_assement.controller.rb文件中显示错误的部分。 我开始的那条线是第36行。因此，第42行有一个问题。

def start
if current_user.taken_any_assessment?.all?
  redirect_to user_root_path
else
  Portal.all.sort_by(&:display_order).each do |portal|
    if !current_user.taken_assessment
(portal.current_assessment(current_user))
      @assessment =       Assessment.find(portal.current_assessment(current_user).id)
      if !@assessment.has_student_taken_this_assessment?(current_user)
        AssessmentResult.create!(student: current_user, assessment: @assessment, last_question: 0)
        @question = @assessment.questions.first
      else
        last_position = AssessmentResult.where(student: current_user, assessment: @assessment).first
        @question = @assessment.questions.where(position: last_position.last_question+1).first
      end