If you create a custom editor in the EditorNeeded event or method override, and don't set the editor's value, an exception is thrown. Reproducible only with DataProvider. To work this around set the custom editor's initial value to a value different than null. Or you can use the events instead of DataProvider.
It should have similar behavior to the one in RadGridView https://docs.telerik.com/devtools/wpf/controls/radgridview/columns/column-groups
UPDATE: The issue occurs in both cases - when populating with data manually and through DataProvider. We will be able to provide a fix for the DataProvider case, as well as the manual popupating if you use TextBox for editor. If populating manually and you need to use a different editor such as DateTimePicker for example, you will need to handle its background on creation.
Available in LIB version 2017.3.1113, it will be also available in the R1 2018 Release.
This component will have the goal to achieve fast operations when dealing with millions of records from a data source. This should be accomplished by decoupling the UI logic from data management operations like loading, filtering, sorting and grouping which should be delegated to the server. Since the grid will not have ItemsSource property, the developer will be responsible for managing the data to/from the server through a given events. Official version of RadVirtualGrid is available from R3 2017 Release of UI for WPF. Check what's new in R3 2017 here: http://www.telerik.com/support/whats-new/wpf/release-history/ui-for-wpf-r3-2017