Flutter Dropdown add elements in a list

I've got a problem regarding the structure of the buttons with flutter

To put it simply I want to Add items in a table, that the user could've pick from a Searchable Dropdown menu.

The problem is that I add in table, without picking from the DropDown first.

Here is how it looks like, as well as the code:

enter image description here

List<Product> products;
 List<Product> selectedProducts;
 bool sort;

 RelatedProductComboBox _selectedPlatformReach;
 List<RelatedProductComboBox> _platformReach =
 RelatedProductComboBox.getRelatedProduct();
 List<DropdownMenuItem<RelatedProductComboBox>> _dropdownPlatformReach;


 @override
 void initState() {
   sort = false;
   selectedProducts = [];
   products = Product.getProducts();

   _dropdownPlatformReach =
       buildDropdownMenuItemsRelatedProduct(_platformReach);
   _selectedPlatformReach = _dropdownPlatformReach[0].value;

   super.initState();
 }

 onSelectedRow(bool selected, Product product) async {
   setState(() {
     if (selected) {
       selectedProducts.add(product);
     } else {
       selectedProducts.remove(product);
     }
   });
 }


 deleteSelected() async {
   setState(() {
     if (selectedProducts.isNotEmpty) {
       List<Product> temp = [];
       temp.addAll(selectedProducts);
       for (Product product in temp) {
         products.remove(product);
         selectedProducts.remove(product);
       }
     }
   });
 }

 SingleChildScrollView dataRelatedProduct() {
   return SingleChildScrollView(
     scrollDirection: Axis.vertical,
     child: Column(
       children: <Widget>[
         DataTable(
           sortAscending: sort,
           sortColumnIndex: 0,
           columns: [
             DataColumn(
               label: Text("Name"),
               numeric: false,
             ),
           ],
           rows: products
               .map(
                 (product) => DataRow(
                   selected: selectedProducts.contains(product),
                   onSelectChanged: (b) {
                     print("Onselect");
                     onSelectedRow(b, product);
                   },
                   cells: [
                     DataCell(
                       SearchableDropdown(
                         isExpanded: true,
                         value: _selectedPlatformReach,
                         items: _dropdownPlatformReach,
                         isCaseSensitiveSearch: false,
                         onChanged:
                             (RelatedProductComboBox selectedPlatformReach) {
                           setState(() {
                             _selectedPlatformReach = selectedPlatformReach;
                           });
                         },
                       ),
                     ),
                   ],
                 ),
               )
               .toList(),
         ),
       ],
     ),
   );
 }