Try adding to the binding TargetNullValue='', worked for me
<telerik:RadNumericUpDown Value="{Binding Path=Contents, Mode=TwoWay, TargetNullValue=''}"
IsEnabled="True"
IsEditable="True"
IsInteger="True"
NullValue=""
MinWidth="70"
HorizontalAlignment="Left"
HorizontalContentAlignment="Left" />