Datagrid throws Value cannot be null. (Parameter 'source')

Has been working good - nothing changed and suddenly this error notification.
Even refactoring the razor page no change

<div class="col-md-12">
    <RadzenDataGrid @ref="grid0" ColumnWidth="200px" AllowFiltering="true" FilterMode="FilterMode.Advanced" AllowPaging="true" AllowSorting="true"
                    Data="@vertrag" Count=count LoadData=@Grid0LoadData TItem="ImpulsStammdaten.Server.Models.AntragVertrag.Vertragkomplett" RowDoubleClick="@EditRow">

I'm afraid that we will unable to reproduce the exception using the code you've provided. Please post runnable code.