"I was able to reproduce the issue you have. However the reason why you get this error is because the element is not properly identified. In order to do so, please use the "HelpText" search property to help you with the identification. This is required for the CodedUI level 2 and 3 support.
For instance this is a sample test method using your own code as a start point:
I tried with a test application having two buttons with different AutomationIDs, so in case you have more buttons you will easily be able to distinguish them."
public void CodedUITestMethod2()
WpfWindow mainWindow = new WpfWindow();
mainWindow.SearchProperties[WpfControl.PropertyNames.Name] = "MainWindow";
WpfRadButton button1 = new WpfRadButton(mainWindow);
button1.SearchProperties["AutomationId"] = "Button_Test";
button1.SearchProperties["HelpText"] = "RadButton";
WpfRadButton button2 = new WpfRadButton(new UIMainWindowWindow());
button2.SearchProperties["AutomationId"] = "Button_Test2";
button2.SearchProperties["HelpText"] = "RadButton";
double height1 = button1.ActualHeight;
double height2 = button2.ActualHeight;