Opening datagrid context menu from code-behind causes NULL reference exception when index is -1

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C#

WPF

DataGrid

context-menu

C#, wpf problem:

I have a data grid with a context menu. On the `previewmouserightbuttonup` event on the `datagrid`, I set the `contextmenu` placement target to the `datagrid`, set the `contextmenu` datacontext to the view model, and open the context menu.

Everything works when selected index of the data grid is >= 0, but when `selectedindex` of the datagrid (the sender of the mouse event) is -1 I receive a `NULL reference exception` for the context menu.

How do I open the `contextmenu` when selectedindex is -1.

What I have tried:

Codebehind

void Datagrid_PreviewMouseRightButtonUp(object sender, MouseEventArgs e)
      {
       var datagrid = sender as DataGrid;
       datagrid.ContextMenu.DataContext = this.DataContext;
       datagrid.ContextMenu.PlacementTarget = datagrid;
       datagrid.ContextMenu.IsOpen = true;
       e.Handled = true;
       }

XAML

I have the following set on the datagrid to handle the PreviewMouseRightButtonUp event:

PreviewMouseRightButtonUp = "Datagrid_PreviewMouseRightButtonUp"