我正在嘗試建立一個網站，提供上傳您自己的課程的功能。課程結構    Name of course    |-Module1      |-Lecture1      |-Lecture2    |-Module2      |-Lecture1      |-Lecture2使用 Angular 我正在嘗試創建一個動態表單，它將在課程中添加/刪除模塊并在模塊中進行講座到目前為止，我已經寫了以下內容 -課程上傳.component.tsexport class CourseUploadComponent implements OnInit {    courseUploadForm: FormGroup;    constructor(private formBuilder: FormBuilder) { }    ngOnInit() {        this.courseUploadForm = this.formBuilder.group({            coursename: ['', Validators.required],            modules: this.formBuilder.array([                this.initModules()            ])        })    }    initModules() {        return this.formBuilder.group({            modulename: ['', Validators.required],            lectures: this.formBuilder.array([                this.initLecture()            ])        });    }    initLecture() {        return this.formBuilder.group({            lecturename: ['', Validators.required],            description: ['', Validators.required],            lecture: ['', Validators.required]        });    }    addModule() {        const control = <FormArray>this.courseUploadForm.get('modules');        control.push(this.initModules());    }    addLecture() {        const control = <FormArray>this.courseUploadForm.get('lectures');        control.push(this.initLecture());    }    removeModule(i: number) {        const control = <FormArray>this.courseUploadForm.get('modules');        control.removeAt(i);    }     removeLecture(i: number) {        const control = <FormArray>this.courseUploadForm.get('lectures');        control.removeAt(i);    }    getModulesControls(i: number) {        >>>> return [(this.courseUploadForm.controls.modules as FormArray).controls[i]['controls']];    }    getLecturesControls(i: number) {        return [(this.courseUploadForm.controls.lectures as FormArray).controls[i]['controls']];    }}我用 ** > ** 突出顯示了引發錯誤的行一些幫助？