To reproduce:
- Add a spreadsheet and add ribbon bar from the smart tag. 
- Start the application.

Workaround:
protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
    var parameter = 11;
    this.radSpreadsheet1.SpreadsheetElement.CommandDescriptors.SetFontSize.Command.Execute(parameter);

    FontFamilyInfo parameter1 = new FontFamilyInfo(new System.Windows.Media.FontFamily("Calibri"));
    this.radSpreadsheet1.SpreadsheetElement.CommandDescriptors.SetFontFamily.Command.Execute(parameter1);
}