DropDownButton Error {Either zero or 2 or more [DropdownMenuItem]s were detected with the same value)}

i am trying to make dropdownbutton with using my CustomerModel model instances but i am getting same error again and again . i tried override == operator and add hashcode method for that class but it didn’t fix that error . What can i do for fix this error ?

 Expanded(
                                  child: Container(
                                    padding: const EdgeInsets.symmetric(
                                        horizontal: 10),
                                    decoration: BoxDecoration(
                                        color: Colors.white,
                                        borderRadius: BorderRadius.circular(5),
                                        border: Border.all(color: softColor)),
                                    child: DropdownButton<CustomerModel?>(
                                      dropdownColor: Colors.white,
                                      onChanged: (CustomerModel? value) {
                                        setState(
                                          () {
                                            selectedCustomer = value!;
                                          },
                                        );
                                      },
                                      borderRadius: BorderRadius.circular(5),
                                      isExpanded: true,
                                      icon: Icon(
                                        Icons.arrow_drop_down,
                                        color: softColor,
                                      ),
                                      value:
                                          selectedCustomer ?? allCustomers[0],
                                      items: List.generate(
                                        allCustomers.length,
                                        (index) =>
                                            DropdownMenuItem<CustomerModel>(
                                          child: CustomTextBody(
                                            text:
                                                allCustomers[index].companyName,
                                          ),
                                        ),
                                      ),
                                    ),
                                  ),
                                ),

i tried create dropdownbutton with my class instances but i got error

>Solution :

DropdownMenuItem is missing value for your case.

  DropdownMenuItem(
    value: allCustomers[index], //this one