System.Windows.Data Error: 6 : 'ObjectSourceConverter' converter failed to convert value '<null>' (type '<null>'); fallback value will be used, if available. BindingExpression:Path=SelectedItem; DataItem='ComboBox' (Name='ChosenFontSize'); target element is 'MenuItem' (Name=''); target property is 'FontSize' (type 'Double') NotSupportedException:'System.NotSupportedException: DoubleConverter cannot convert from (null).
at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)