Xamarin Forms Listview: Deselecting selected item by checking with OnTapped Event

I will be able to deselect my item when i click second time on it. When I tap an item the background color changed to blue. I want that the background gets the default color when I tap second time to the same item from the list. Here can you find my code but it isn't working.

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I will be able to deselect my item when i click second time on it. When I tap an item the background color changed to blue. I want that the background gets the default color when I tap second time to the same item from the list. Here can you find my code but it isn't working.

XAML

<StackLayout Margin="5,0,5,0" HorizontalOptions="FillAndExpand" VerticalOptions="CenterAndExpand">
<Frame Style="{StaticResource StandardFrameStyle}" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
  <ListView x:Name="lst_pickList" ItemSelected="OnItemSelected" ItemTapped="OnItemTapped" ItemsSource="{Binding PickList}" HasUnevenRows="True" Style="{StaticResource StandardListViewStyle}">
    <ListView.ItemTemplate>
      <DataTemplate>
        <ViewCell>
          <ViewCell.View>
            <Grid x:Name="grd_pickList" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" >
              <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
              </Grid.RowDefinitions>
              <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*" />
                <ColumnDefinition Width="*" />
                <ColumnDefinition Width="*" />
                <ColumnDefinition Width="*" />
              </Grid.ColumnDefinitions>

              <Label x:Name="lbl_comment" Text="Comment:" FontAttributes="Bold" Grid.Row="0" Grid.Column="0" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
              <Label x:Name="lbl_itemNo" Text="ItemNo:" FontAttributes="Bold" Grid.Row="0" Grid.Column="2" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
              <Label x:Name="lbl_description" Text="Description:" FontAttributes="Bold" Grid.Row="1" Grid.Column="0" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
              <Label x:Name="lbl_restant" Text="Restant:" FontAttributes="Bold" Grid.Row="1" Grid.Column="2" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />

              <Label x:Name="lbl_comment_binding" Text="{Binding Comment}" Grid.Row="0" Grid.Column="1" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
              <Label x:Name="lbl_itemNo_binding" Text="{Binding ItemNo}" Grid.Row="0" Grid.Column="3" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
              <Label x:Name="lbl_description_binding" Text="{Binding Description}" Grid.Row="1" Grid.Column="1" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
              <Label x:Name="lbl_restant_binding" Text="{Binding Restant}" Grid.Row="1" Grid.Column="3" VerticalTextAlignment="Center" Style="{StaticResource StandardLabelStyle}" />
            </Grid>
          </ViewCell.View>
        </ViewCell>
      </DataTemplate>
    </ListView.ItemTemplate>
  </ListView>
</Frame>

CodeBehind

public void OnItemTapped(object sender, ItemTappedEventArgs args)
{
    var newSelectedPick = args.Item as Pick;

    if(selectedPick != null && selectedPick.Id == newSelectedPick.Id)
    {
        lst_pickList.SelectedItem = null;
    }
    else
    {
        selectedPick = newSelectedPick;
    }
}