Fix potential race condition, addresses CommunityToolkit#616

Author: Arlodotexe

components/MarkdownTextBlock/src/MarkdownTextBlock.xaml.cs

@@ -88,8 +88,9 @@ private void ApplyText(bool rerender)
 
             if (!string.IsNullOrEmpty(Text))
             {
-                this.MarkdownDocument = Markdown.Parse(Text, _pipeline);
-                _renderer.Render(this.MarkdownDocument);
+                var parsedMarkdown = Markdown.Parse(Text, _pipeline);
+                this.MarkdownDocument = parsedMarkdown;
+                _renderer.Render(parsedMarkdown);
             }
         }
     }