지금 과제로 앱을 만들고 있는데, 추가하려는 기능 중에 새로운 창에서 버튼을 누르면 기존 창에 버튼을 추가시키는 형태로 만들고 있습니다.

그런데 리니어 레이아웃이 Null이 반환되어서 오류가 납니다.

어떻게 해야 오류가 안 나게 작성할 수 있을까요

`public class create_file_Activity extends AppCompatActivity {

Context context;
LinearLayout main_layout;

EditText create_file_title, create_file_content;
Button create_file_btn_date, create_file_btn_time, create_file_btn_cancle, create_file_btn_confirm;

TextView time_tv, date_tv;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.create_file);
    setTitle("일정 추가");

    create_file_title = (EditText) findViewById(R.id.create_file_title);
    create_file_content = (EditText) findViewById(R.id.create_file_content);

    create_file_btn_date = (Button) findViewById(R.id.create_file_btn_date);
    create_file_btn_time = (Button) findViewById(R.id.create_file_btn_time);
    create_file_btn_cancle = (Button) findViewById(R.id.create_file_btn_cancle);
    create_file_btn_confirm = (Button) findViewById(R.id.create_file_btn_confirm);

    main_layout = (LinearLayout) findViewById(R.id.Mainlayout);

    time_tv = (TextView) findViewById(R.id.time_tv);
    date_tv = (TextView) findViewById(R.id.date_tv);

    context = this;

    create_file_btn_cancle.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent = new Intent(getApplicationContext(), MainActivity.class);
            startActivity(intent);
        }
    });

    create_file_btn_confirm.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            Button btn = new Button(context);
            btn.setText(create_file_title.getText().toString());
            btn.setWidth(LinearLayout.LayoutParams.MATCH_PARENT);

            main_layout.addView(btn);
        }
    });

    create_file_btn_time.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            Calendar now = Calendar.getInstance();

            final TimePickerDialog tpDialog = new TimePickerDialog(create_file_Activity.this, new TimePickerDialog.OnTimeSetListener() {
                @Override
                public void onTimeSet(TimePicker timePicker, int selectedHour, int selectedMinute) {
                    String state = "AM";
                    if (selectedHour > 12) {
                        selectedHour -= 12;
                        state = "PM";
                    }
                    time_tv.setText(state + " " + selectedHour + "시 " + selectedMinute + "분");
                }
            }, now.get(Calendar.HOUR_OF_DAY), now.get(Calendar.MINUTE), false); // true의 경우 24시간 형식의 TimePicker 출현
            tpDialog.getCurrentFocus();
            tpDialog.show();
        }
    });

    create_file_btn_date.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            DatePickerDialog dpDialog = new DatePickerDialog(create_file_Activity.this, new DatePickerDialog.OnDateSetListener() {
                @Override
                public void onDateSet(DatePicker datePicker, int i, int i1, int i2) {
                    date_tv.setText(i+"년 "+(i1+1)+"월 "+i2+"일");
                }
            }, 0, 0, 0);
            dpDialog.getDatePicker().setMinDate(System.currentTimeMillis());
            dpDialog.show();
        }
    });
}

}`