NoMethodError: undefined method `env' for nil:NilClass

I am using rails testing for my app and when I run this test

class ShowingQuizzesTest < ActionDispatch::IntegrationTest
    include Devise::TestHelpers
    setup { host! 'example.com' }
    test 'return quiz by id' do 
        instructor=Instructor.create(name:"same7", email:"abcdefg@eng.asu.edu.com", password:"123abc")
        sign_in instructor
        quiz = Quiz.create(name: 'Quiz1', subject: 'physics', duration: 10, no_of_MCQ: 5, no_of_rearrangeQ: 5) 
        current_instructor.quizzes << quiz
        get "api/quizzes/#{quiz.id}"
        assert_equal 200, response.status

        quiz_response = json(response.body)
        assert_equal quiz.name, quiz_response[:name]
    end
end     

This error appears: 1) Error: ShowingQuizzesTest#test_return_quiz_by_id: NoMethodError: undefined method `env' for nil:NilClass