Flutter - There should be exactly one item with [DropdownButton]'s value... error with provider

I have a dropdown-menu. My goal is to let the user choose a value of the dropdown-menu. After this the value should be shown as a list above the dropdown-menu. I'm using Provider for storing the choosen value(s) in a List. But when I let the List listen to the changes in the provider-model I get the error for the Dropdown-menu. I don't know why I get errors during rebuilding...

Provider.of<InspectionFormModel>(context, listen: true).removeAbleObjects!=null?ListView(
                      shrinkWrap: true,
                      key: ObjectKey(Provider.of<InspectionFormModel>(context, listen: false).removeAbleObjects),
                      children: Provider.of<InspectionFormModel>(context, listen: false).removeAbleObjects!=null?Provider.of<InspectionFormModel>(context, listen: false).removeAbleObjects!.map((e) => e).toList():[Container()]
                    ):Visibility(child: Container(),visible: false,),
                    CustomDropdown(items: [DropDownObject("text",1),DropDownObject("textsoundso",2)], hintText: inspectionObject, provider: "inspection",value:"device",),

My DropDownButton:

import 'package:dropdown_button2/dropdown_button2.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';


class CustomDropdown extends StatefulWidget {
  final List<DropDownObject> items;
  //final List<dynamic>? objects;
  final String hintText;
  final String? provider;
  final String? value;
  final bool useId;

  const CustomDropdown({Key? key,
    this.useId=false,
    this.value,
    this.provider,
    this.hintText="",
    required this.items}) : super(key: key);

  @override
  State<CustomDropdown> createState() => _CustomDropdownState();
}

class _CustomDropdownState extends State<CustomDropdown> {


  @override
  void initState() {
    developer.log("init dropdown");
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    DropDownObject? selectedValue=null;
  


    return Padding(
      padding: EdgeInsets.only(right: 15),
        // child: DropdownButtonHideUnderline(
          child: DropdownButtonFormField2<DropDownObject>( 

            validator: (value) {
              if (value == null ) {
                return 'Bitte wählen Sie einen Wert aus.';
              }
              return null;
            },
            buttonHeight: 50,
            decoration: InputDecoration(
              // errorBorder: OutlineInputBorder(borderSide: BorderSide(color: Colors.red),borderRadius: BorderRadius.circular(10)),
                enabledBorder: UnderlineInputBorder(
                    borderSide: BorderSide(color: Colors.transparent)
                )
            ),//InputDecoration.collapsed(hintText: ""),//
            hint: Padding(padding: EdgeInsets.only(left: 10),child: Text(widget.hintText, style: AppTextStyle().formText,)),
            isExpanded: true,
            dropdownDecoration: BoxDecoration(
              borderRadius: BorderRadius.circular(10),
              border: Border.all(
                color: AppColors().grey,
              ),
            ),
            buttonDecoration: BoxDecoration(
              color: AppColors().formFieldColor,
              borderRadius: BorderRadius.circular(10),
              border: Border.all(
                color: AppColors().grey,
              ),
            ),
            items: widget.items.map((item) =>
                DropdownMenuItem<DropDownObject>(
                  value: item,
                  child: Row(children:[
                    Expanded(child: Text(item.text, textAlign: TextAlign.left,))
                  ],)
                )).toList(),
            // value:selectedValue,
            onChanged: (value) {
              setState(() {
                selectedValue=value;
                //selectedValue = value;
                //widget.useId?widget.provider??FormProviderSelection().writeValueToProviderObject(context, widget.provider!,widget.value!,selectedValue!):
                FormProviderSelection().writeValueToProviderObject(context, widget.provider!,widget.value!,value!);//widget.provider??
              });
            },
            icon: const Icon(Icons.arrow_drop_down_rounded),
            iconEnabledColor: AppColors().grey,
            iconDisabledColor: Colors.white54,
            dropdownMaxHeight: 200,
            dropdownElevation: 8,
          ),
        // ),
    );
  }
}

Update 2022-01-22

class DropDownObject{
  String text;
  int? dropDownObjectId;

  DropDownObject(this.text, [this.dropDownObjectId]);
}

Solution 1:

Maybe you have two or more same DropDownObject. Or you have null DropDownObject.

Check this part of your code.

items: widget.items.map((item) =>
                DropdownMenuItem<DropDownObject>(
                  value: item,
                  child: Row(children:[
                    Expanded(child: Text(item.text, textAlign: TextAlign.left,))
                  ],)
                )).toList(),

try to add filters:

widget.items
.where((e) => e != null) //removes null items
.toSet() // removes duplicate items
.map

also implement == and hashcode in your model class:

class DropDownObject{
  String text;
  int? dropDownObjectId;

  DropDownObject(this.text, [this.dropDownObjectId]);

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
          other is DropDownObject && runtimeType == other.runtimeType && text == other.text;

  @override
  int get hashCode => text.hashCode;


}