当我尝试将一组标记分配给 GoogleMap 中的标记参数时，它给了我这个错误...

代码示例

body: GoogleMap(
    initialCameraPosition: CameraPosition(
      target: LatLng(
        widget.initialLocation!.latitude!,widget.initialLocation!.longitude!,),zoom: 16,onTap: widget.isSelecting! ? _selectLocation : null,markers: _pickedLocation == null
        ? null
        : {
            Marker(
              markerId: MarkerId('m1'),position: _pickedLocation!,},