One way to work around this is to include a dummy resource with the name of "HexagonalSevenSegsN"
<telerik:RadVerticalLinearGauge>
 <FrameworkElement.Resources>
  <DataTemplate x:Key="HexagonalSevenSegsN" />

The same exception is also thrown when you are trying to bind the NumericIndicator Value property when using SevenSegsNumberPosition position. In this scenario, the data context is not propagated to the NumericIndicator. To workaround this you can set the Source binding.
<Window.Resources>
     <local:TestViewModel x:Key="viewModel"/>
</Window.Resources>

<tk:RadRadialGauge Name="gauge" Height="200" Width="200" Margin="5">
            <tk:RadialScale Min="0" Max="300" MajorTicks="10" LabelRotationMode="None">
                <tk:RadialScale.Indicators>
                    <tk:NumericIndicator  x:Name="numericIndicator" Value="{Binding TestValue,Source={StaticResource viewModel}}"

*** The fix for this issue will be available with the next LIB (version 2018.3.1217) expected on Monday, December 17.