WPF - Throw exception if Load called before browser initialized

Follow up to #2260

Author: amaitland

CefSharp.Wpf/ChromiumWebBrowser.cs

@@ -2157,6 +2157,12 @@ private void OnMouseButton(MouseButtonEventArgs e)
         /// <param name="url">The URL to be loaded.</param>
         public void Load(string url)
         {
+            if (!InternalIsBrowserInitialized())
+            {
+                throw new Exception("The browser has not been initialized. Load can only be called" +
+                                    "after the underlying CEF browser is initialized (CefLifeSpanHandler::OnAfterCreated).");
+            }
+
             // Added null check -> binding-triggered changes of Address will lead to a nullref after Dispose has been called
             // or before OnApplyTemplate has been called
             if (browser != null)