null 不能转换为非 null 类型 com.example.kotlindiary.MainActivity

在主activity的fragment内部，当从FireBase加载数据时，会检查用户是否拥有所有必要的数据，如果没有，则将他发送到所需的activity输入数据。当用户到达所需的活动时，他可以在不同的活动之间切换以输入不同的信息（学校->班级->时间表）

并且由于某种原因，在与学校的活动中，在选择学校时，会弹出一个错误，该错误位于主要活动的片段中。

kotlin.TypeCastException: null cannot be cast to non-null type com.example.kotlindiary.MainActivity
        at com.example.kotlindiary.MainFragment$downloadHomework$1$onDataChange$1.onDataChange(MainFragment.kt:210)

抱怨 Intent 初始化的行：

if(timetableisreal){...}
else{
    val intent = Intent(activity, SetTimetableActivity::class.java)
    intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK.or(Intent.FLAG_ACTIVITY_CLEAR_TASK)
    startActivity(intent)
}

我注意到选择学校时会弹出错误，并将错误发送到主片段。我假设 ValueEventListener 多次运行此代码，此时发生错误。但是奇怪的是，在主要活动完成后，监听器继续工作。提前致谢

UPD：片段代码

class MainFragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?

    ): View? {
        ...
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        ...
        downloadHomework()
    }

    override fun onCreateOptionsMenu(menu: Menu, inflater: MenuInflater) {
        super.onCreateOptionsMenu(menu, inflater)
    }

    fun downloadHomework(){
        ...
        ref.addValueEventListener(object : ValueEventListener{
            override fun onDataChange(p0: DataSnapshot) {
                user = p0.getValue(User::class.java)
                val schoolName = user?.school
                val form = user?.form
                if(user?.name == "" || user?.surname == ""){//проверка, есть ли имя, фамилия
                    val intent = Intent(activity, ProfileActivity::class.java)
                    intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK.or(Intent.FLAG_ACTIVITY_CLEAR_TASK)
                    startActivity(intent)
                    (activity as MainActivity).finish()
                }
                else if(schoolName == ""){//выбрана ли школа
                    val intent = Intent(activity, ChooseSchoolActivity::class.java)
                    intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK.or(Intent.FLAG_ACTIVITY_CLEAR_TASK)

                    startActivity(intent)
                    (activity as MainActivity).finish()
                }
                else if(form == ""){//выбран ли класс
                    val intent = Intent(activity, ChooseFormActivity::class.java)
                    intent.putExtra("schoolName", schoolName)
                    intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK.or(Intent.FLAG_ACTIVITY_CLEAR_TASK)
                    startActivity(intent)
                    (activity as MainActivity).finish()
                }
                else{
                    val refTimetable = FirebaseDatabase.getInstance().getReference("/schools/$schoolName/$form/timetable")
                    refTimetable.addValueEventListener(object : ValueEventListener{
                        override fun onDataChange(p0: DataSnapshot) {
                            p0.children.forEach {
                                //проверка, есть ли расписание
                            }
                            if(timetableisreal){...}
                            else{
                                val intent = Intent(activity, SetTimetableActivity::class.java)//если нет, отправляем на активити с установкой расписания
                                intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK.or(Intent.FLAG_ACTIVITY_CLEAR_TASK)
                                startActivity(intent)
                            }

                        }
                        override fun onCancelled(p0: DatabaseError) {}
                    })
                }

            }
            override fun onCancelled(p0: DatabaseError) {}
        })
    }
}