I am trying to get grouping in my datagrid. I have found interesting idea on Stack Overflow, here: /a/43044022/5128696 . I sort of adapted the code to my project, here is XAML:

<UserControl.Resources >
    <CollectionViewSource x:Key="persons" Source="{Binding Path=WorkersTasksGridTableSource}">
        <CollectionViewSource.GroupDescriptions>
            <PropertyGroupDescription PropertyName="DayOfRecord" />
        </CollectionViewSource.GroupDescriptions>
    </CollectionViewSource>
</UserControl.Resources>

and later in same file:

<DataGrid x:Name="WorkerTasksDataGrid" AutoGenerateColumns="False" HorizontalGridLinesBrush="DarkGray" VerticalGridLinesBrush="DarkGray" 
          Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="6" ItemsSource="{Binding Source={StaticResource persons}}" >
    <!-- /a/43044022/5128696 -->            
    <DataGrid.Columns>
        <DataGridTextColumn Header="Start Date" Binding="{Binding Path=StartDate, StringFormat={0:dd.MM.yy HH:mm:ss}}"  />
        <DataGridTextColumn Header="End Date" Binding="{Binding Path=EndDate, StringFormat={0:dd.MM.yy HH:mm:ss}}"  />
        <DataGridTextColumn Header="Hours" Binding="{Binding Path=CalculatedTime}"  />
        <DataGridTextColumn Header="Project" Binding="{Binding Path=Project}"  />
        <DataGridTextColumn Header="Task" Binding="{Binding Path=Task}"  />
        <DataGridTextColumn Header="Remark" Binding="{Binding Path=Remark}"  />
        <DataGridTextColumn Header="Target Company, Customer, Project" Binding="{Binding Path=TargetCompany}"  />
    </DataGrid.Columns>

    <DataGrid.GroupStyle>
        <GroupStyle>
            <GroupStyle.ContainerStyle>
                <Style TargetType="{x:Type GroupItem}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type GroupItem}">
                                <Expander IsExpanded="True" >
                                    <Expander.Header>
                                        <DockPanel>
                                            <TextBlock Text="{Binding Path=Name}" />
                                        </DockPanel>
                                    </Expander.Header>
                                    <Expander.Content>
                                        <ItemsPresenter />
                                    </Expander.Content>
                                </Expander>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </GroupStyle.ContainerStyle>
        </GroupStyle>
    </DataGrid.GroupStyle>

</DataGrid>

Real ItemSource Observable Collection WorkersTasksGridTableSource is being created in my user control constructor . Groups are created finely but I get this cryptic binding error:

Severity|Count|Data Context|Binding Path  |Target             |Target Type |Description 
Error   |1    |null        |NewItemMargin |DataGridRow.Margin |Thickness   |Cannot find source: RelativeSource FindAncestor, AncestorType='System.Windows.Controls.DataGrid', AncestorLevel='1'.

I am not using NewItemMargin at all! Why am I getting this error and how to overcome such error?