How to binding Handler in datagrid with command?

Question

I want to binding handler in EventSetter so my code till now

<DataGrid  IsReadOnly="True" x:Name="OverviewDatagrid" ItemsSource="{Binding MyCollection}"
                  AutoGenerateColumns="False"
                  CanUserAddRows="False">
            <DataGrid.Resources>
                <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" 
                   Color="#404040"/>
            </DataGrid.Resources>
            <DataGrid.CellStyle>
                    <Style TargetType="DataGridCell">
                        <EventSetter Event="MouseDown" Handler="DataGridCell_MouseDoubleClick"/>
                    </Style>
                </DataGrid.CellStyle>
            <DataGrid.Columns>

                <DataGridTextColumn Width="300" Header="FirstName" Binding="{Binding Code, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
                <DataGridTextColumn Header="LastName" Binding="{Binding LastName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

            </DataGrid.Columns>
        </DataGrid>

When i try to binding Handler it return me exception

 <EventSetter Event="MouseDown" Handler="{Binding ClickCommand}"/>

So ViewModel.cs

 private ICommand _clickCommand;
        public ICommand ClickCommand
        {
            get
            {
                return _clickCommand ?? (_clickCommand = new CommandHandler(() => MyAction(), () => CanExecute));
            }
        }
        public bool CanExecute
        {
            get
            {
                // check if executing is allowed, i.e., validate, check if a process is running, etc. 
                return true;
            }
        }

        public void MyAction()
        {
        }

And CommandHandler.cs file

public class CommandHandler : ICommand
    {
        private Action _action;
        private Func<bool> _canExecute;

        /// <summary>
        /// Creates instance of the command handler
        /// </summary>
        /// <param name="action">Action to be executed by the command</param>
        /// <param name="canExecute">A bolean property to containing current permissions to execute the command</param>
        public CommandHandler(Action action, Func<bool> canExecute)
        {
            _action = action;
            _canExecute = canExecute;
        }

        /// <summary>
        /// Wires CanExecuteChanged event 
        /// </summary>
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        /// <summary>
        /// Forcess checking if execute is allowed
        /// </summary>
        /// <param name="parameter"></param>
        /// <returns></returns>
        public bool CanExecute(object parameter)
        {
            return _canExecute.Invoke();
        }

        public void Execute(object parameter)
        {
            _action();
        }
    }

I am new to c# and i dont know why it return me This line return error.

>  <EventSetter Event="MouseDown" Handler="{Binding ClickCommand}"/>

System.NullReferenceException: 'Object reference not set to an instance of an object.'