How to save elements in a database container with Angular?

I'm working with Angular, and I need some advice. I tried to create a container, where it's possible to drop boxes with names like dropdown or calendar. These boxes come with attributes, like a pink calendar or a dropdown containing 3 items.

What I want is, for these names with attributes, to be saved in my database. I have created a REST API, but I don't really know how to save such things. I tried to drop these elements into an input, so I can save them with formControlName, but it's not possible to drop elements into a input. I also tried using a ngSwitch, where every ngSwitchCase has its own formControlName, but then I get the error:

Cannot find control with path: 'forms -> 0 -> Textbox.0'

formadd.component.html

div class="add">
  <form [formGroup]="paginaForm" (submit)="onFormSubmit()">
    <div formArrayName="forms">
      <div class="drag" *ngFor="let form of forms; let i = index;" [formGroupName]="i">
        <mat-toolbar class="toolbar" color="warn">
          <div class="title">
            <input class="form_name" placeholder="Formulier Naam" type="text" formControlName="form_name" #form_name />
          </div>
          <div
            *ngIf="paginaForm.controls['form_name']?.invalid && (paginaForm.controls['form_name'].dirty || paginaForm.controls['form_name'].touched)"
            class="alert alert-danger">
            <div *ngIf="paginaForm.controls['form_name']?.errors.required">
              Formuliernaam is required.
            </div>
          </div>
        </mat-toolbar>
        <fieldset class="element">
          <legend class="voeg-element" placeholder="voeg element toe" cdkDropList
          #doneList="cdkDropList"
          [cdkDropListData]="done"
          (cdkDropListDropped)="drop($event)">
          <div [ngSwitch]="item" class="cdkdrag" *ngFor="let item of done; let i = index" cdkDrag>
            <div *ngSwitchCase="'Categorie'">{{item}}</div>
            <div *ngSwitchCase="'Textbox'">
              <input type="text" [formControlName]="item + '.' + i" #element/> 
            </div>
            <div *ngSwitchCase="'Textarea'">
              <textarea kendoTextArea></textarea>
            </div>
            <div *ngSwitchCase="'Date'">
                <div class="example-wrapper">
                    <p>Selecteer een datum:</p>
                    <kendo-dateinput
                [(value)]="value"></kendo-dateinput>
                </div>
            </div>
            <div *ngSwitchCase="'Email'">{{item}}</div>
            <div *ngSwitchCase="'Integer'">{{item}}</div>
            <div *ngSwitchCase="'Boolean'">{{item}}</div>
            <div *ngSwitchCase="'Checkbox'">{{item}}</div>
            <div *ngSwitchCase="'Dropdown'">{{item}}</div>
            <div *ngSwitchCase="'Label'">{{item}}</div>
            <div *ngSwitchCase="'Locatie'">{{item}}</div>
            <div *ngSwitchCase="'Subject'">{{item}}</div>
            <div *ngSwitchCase="'Document'">{{item}}</div>
            <div *ngSwitchDefault>{{item}}</div>
            </div>
        </legend>
        <div
          *ngIf="paginaForm.controls['element']?.invalid && (paginaForm.controls['element'].dirty || paginaForm.controls['element'].touched)"
          class="alert alert-danger">
          <div *ngIf="paginaForm.controls['element']?.errors.required">
            Element is required.
          </div>
        </div>
        <div class="verwijder">
          <button (click)="removePagina(i)">Verwijder Pagina</button>

        </div>
        <div class="opslaan">
          <button (click)="addFolders(form_name.value, element.value)"
            [disabled]="paginaForm.pristine || paginaForm?.invalid">Opslaan Pagina</button>

        </div>
      </fieldset>
      </div>
    </div>
    <div class="toevoegen">
      <button type="button" (click)="addPagina()">
        Pagina toevoegen
      </button>
    </div>
  </form>
</div>

formadd.component.ts

import { Component, OnInit } from '@angular/core';
import { FormGroup, Validators, FormBuilder, FormArray } from '@angular/forms';
import { FoldersService } from '../folders.service';
import { CdkDragDrop, moveItemInArray, transferArrayItem } from '@angular/cdk/drag-drop';

@Component({
  selector: 'app-formadd',
  templateUrl: './formadd.component.html',
  styleUrls: ['./formadd.component.css']
})
export class FormaddComponent implements OnInit {
  paginaForm: FormGroup;

  constructor(private fb: FormBuilder, private bs: FoldersService) {
    this.createForm();
  }


  createForm() {
    this.paginaForm = this.fb.group({
      form_name: ['', Validators.required],
      element: ['', Validators.required],
    });
  }

  addFolders(form_name, element) {
    console.log(this.paginaForm.value.forms[0]);
    this.bs.addFolders(form_name, element);
  }

  ngOnInit() {
    this.paginaForm = this.fb.group({
      forms: this.fb.array([])
    });
  }

  onFormSubmit() {
    alert(JSON.stringify(this.paginaForm.value));
  }

  addPagina() {
    (<FormArray>this.paginaForm.get('forms')).push(this.fb.group({
      form_name: [],
      element: [],
    }));
  }

  get forms() {
    return (<FormArray>this.paginaForm.get('forms')).controls;
  }

  removePagina(i: number) {
    (<FormArray>this.paginaForm.get('forms')).removeAt(i);
  }

  done = [];
  drop(event: CdkDragDrop<string[]>) {
    console.log(event);
    this.done.push(event.item.data);

or

Cannot read property 'value' of undefined

When I change the statement

<div [ngSwitch]="item" class="cdkdrag" *ngFor="let item of done; let i = index" cdkDrag>
            <div *ngSwitchCase="'Categorie'">{{item}}</div>
            <div *ngSwitchCase="'Textbox'">
              <input type="text" [formControlName]="item + '.' + i" #element/> 
            </div>

to

<div [ngSwitch]="item" class="cdkdrag" *ngFor="let item of done; let i = index" cdkDrag>
            <div *ngSwitchCase="'Categorie'">{{item}}</div>
            <div *ngSwitchCase="'Textbox'">
              <input type="text" [formControlName]="element" #element/> 
            </div>

Does anyone know a better way to figure out this super task?