Learning WPF, coming from WinForm experience. I’m working on converting a WinForm program that uses a DataGridView to display (text) in multiple columns with project search results. The DataGridView is 5 columns 4 of which is text and the first column is integer.
I have created in the new WPF project a DataGridView mimicking my WinForm project as best as I can reason

<DataGrid x:Name="dataGrid" Grid.Row="3" Grid.Column="5" AutoGenerateColumns="False" IsEnabled="{Binding IsdataGrid}"
      ItemsSource="{Binding GridDataTable}">
<DataGrid.Columns>
    <DataGridTextColumn Header="ID" Binding="{Binding gvID}" Width="30"/>
    <DataGridTextColumn Header="File Name" Binding="{Binding gvFileName}" Width="240"/>
    <DataGridTextColumn Header="Number of Lines" Binding="{Binding gvNumberOfLines}" Width="80" 
                        HeaderStyle="{StaticResource WrappedColumnHeaderStyle}"/>
    <DataGridTextColumn Header="Number of Matches" Binding="{Binding gvNumberOfMatches}" Width="80" 
                        HeaderStyle="{StaticResource WrappedColumnHeaderStyle}"/>
    <DataGridTextColumn Header="Duration" Binding="{Binding gvDuration}" Width="60"/>
</DataGrid.Columns>

I’m invoking the grid population from a button press (AsyncRelayCommand(OnbtnProcessExecute, () => true);) not a button_click method.

My public class of

    public class DataGridInput
{
    public int ID { get; set; }
    public string gvFileName { get; set; }
    public string gvNumberOfLines { get; set; }
    public string gvNumberOfMatches { get; set; }
    public string gvDuration { get; set; }
}

My grid declaration in the MainwindowsViewModel

    private DataGrid _gridDataTable;
    public DataGrid GridDataTable
    {
        get => _gridDataTable;
        set
        {
            if(_gridDataTable == value)
                return;
            _gridDataTable = value;
            OnPropertyChanged(nameof(GridDataTable));
        }
    }

When the body of the project finds details of a file before processing to the next files I would like to populate a row in DataGridView with the column data like:
Id, Filename, Number of lines, Number of lines, duration
All are in the processing result. In WinForm’s DataGridView it was a matter of doing:
dataGridView1.Rows.Add(result[0], result[1], result[2], result[3]), result[4];

I have search far and near but can’t find information that I can use to help me write to the DataGridView. I have tried

            List<DataGridInput> dgi = new List<DataGridInput>();
            dgi.Add(new DataGridInput()
            {
                ID = 1,
                gvFileName = "Testthis.txt",
                gvNumberOfLines = "20",
                gvNumberOfMatches = "22",
                gvDuration = "00:03:00"
            });
            GridDataTable.ItemsSource = dgi;

But I’m getting “System.NullReferenceException Message=Object reference not set to an instance of an object.”

In short I’m over my head, looking for assistance on how to convert my WinForm.DataGridView to a working WPF model.