========小部件库捕获到异常===================================================================在构建MyHomePage（dirty，dependencies：[MediaQuery]，状态：_MyHomePageState#a3bdf（代码：跟踪1个自动收报机））：应该只有一个项目具有[DropdownButton]的值：标记{markerId：MarkerId（第一个标记），alpha：

1.0，锚：Offset（0.5，1.0），consumeTapEvents：false，可拖动：假，平坦：false，图标：'BitmapDescriptor'的示例，infoWindow：InfoWindow{title：第一个标记，片段：空，锚：偏移（0.5，0.0）}，位置：LatLng（37.42267，-122.0838），旋转：0.0，可见：true，zIndex：0.0，onTap：null，onDragStart：null，onDrag：null，onDragEnd：null}。检测到零个或2个或更多[DropdownMenuItem]具有相同的值'package：flutter/src/material/dropdown. dart'：Assert失败：第888行位置15：

'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'

    DropdownButton(
                    value: selectedMarker,
                onChanged: ((Marker? marker) {
                  setState(() {
                    selectedMarker = marker!;
                  });
                  print("selectedmarker value ${selectedMarker}");
                  mapController.moveCamera(CameraUpdate.newLatLng(marker!.position));
                }),
            items: markers.map((marker) {
              return DropdownMenuItem(
                value: marker,
                child: Text(marker.markerId.value),
              );
            }).toList(),
          ),

这是导致错误的下拉按钮小部件。现在我想实现的事情是，在从菜单中选择一个项目后，Map上的标记被选中。那已经完成了。现在，如果我按下方向按钮，然后标记将遵循polylinepoints。当我按下按钮时，出现此错误。方向按钮代码如下：

IconButton(
   onPressed: () async {
         if(selectedMarker.markerId == MarkerId("First Marker")){
           controller = AnimationController(
            duration: const Duration(milliseconds: 1000),
               vsync: this,
               );
                 selectedMarker = markers[0];
       for (int i = 0; i < polylinePoints.length && i + 1 != polylinePoints.length; i = i + 1) {
                                markerAnimation = LatLngTween(
                                    begin: polylinePoints[i],
                                    end: polylinePoints[i + 1])
                                    .animate(controller);
                                markerAnimation.addListener(() {
                                  setState(() {
          Marker marker1 = Marker(markerId: MarkerId("First Marker",
                       ),icon:BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueGreen),
                                    );
                                    print("This is marker1 ${marker1}");
                                    marker1 = marker1.copyWith(
                                      positionParam: markerAnimation.value,
                                    );
                                    ///will be right back
                                    markers[0] = marker1;
                                    // _addFirstMarker(Variables.point);
                                  });
                                });
                                controller.forward();
                                await Future.delayed(
                                    const Duration(milliseconds: 50));
                              }
                            }
                            if(selectedMarker.markerId == MarkerId("Second Marker")){
                              controller = AnimationController(
                                duration: const Duration(milliseconds: 1000),
                                vsync: this,
                              );
                 selectedMarker = markers[1];
     for (int i = 0; i < polylinePoints1.length && i + 1 != polylinePoints1.length; i = i + 1) {
                                markerAnimation = LatLngTween(
                                    begin: polylinePoints1[i],
                                    end: polylinePoints1[i + 1])
                                    .animate(controller);
                                markerAnimation.addListener(() {
                                  setState(() {
          Marker marker1 = Marker(markerId: MarkerId("Second Marker",
            ),icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueGreen),
                                    );
                                    marker1 = marker1.copyWith(
                                      positionParam: markerAnimation.value,
                                    );
                                    markers[1] = marker1;
                                    // _addFirstMarker(Variables.point);
                                  });
                                });
                                controller.forward();
                                await Future.delayed(
                                    const Duration(milliseconds: 50));
                              }
                            }
                            if(selectedMarker.markerId == MarkerId("Third Marker")){
                              controller = AnimationController(
                                duration: const Duration(milliseconds: 1000),
                                vsync: this,
                              ); selectedMarker = markers[2];
     for (int i = 0; i < polylinePoints2.length && i + 1 != polylinePoints2.length; i = i + 1) {
                                markerAnimation = LatLngTween(
                                    begin: polylinePoints2[i],
                                    end: polylinePoints2[i + 1])
                                    .animate(controller);
                                markerAnimation.addListener(() {
                                  setState(() {
          Marker marker1 = Marker(markerId: MarkerId("Third Marker",
           ), icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueGreen),
                                    );
                                    marker1 = marker1.copyWith(
                                      positionParam: markerAnimation.value,
                                    );
                                    markers[2] = marker1;
                                    // _addFirstMarker(Variables.point);
                                  });
                                });
                                controller.forward();
                                await Future.delayed(
                                    const Duration(milliseconds: 50));
                              }
                            }
                          },
                          icon: Icon(Icons.directions))