This framework error might occur if you are using the RadMap with Visualizationlayer's virtualization inside WinForms control (via ElementHost). Generally, In Winforms you can work with UI controls only from the main UI thread.
In your case, I think the UI controls you use in the method are "_polyline" and "FeaturesList". You can try execute this code in Dispatcher.BeginInvoke() block. Also, if you are using VisualizationLayer (VizLayer), you should work with Map Shape Data objects (which are not UI Controls). The MapPolyline control is typical for InformationLayer and should not be used in VizLayer.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items