How to assign the value of a component link field to an XML element?

Question

 if (currentComponent.Schema.Title == "dtd_link_item") 
 {
      var linkComponentFields = currentComponent.Fields();
      var element = new XElement("SiteMapNode");                    
      element.Add(new XAttribute("uri", currentComponent.Id));                   
      element.Add(new XAttribute("url", currentComponentFields.AsText("linked_url")));
      element.Add(new XAttribute("link", linkComponentFields.Component("linked_component").Id));

      var title = linkComponentFields.AsText("title") != string.Empty ? linkComponentFields.AsText("title") : linkComponentFields.AsText("link_title");
      title = title != string.Empty ? title : GetTitle(linkComponentFields);
      element.Add(new XAttribute("title", title));

      xRoot.Add(element);
  }

This is how I am assigning it, but Template Builder gives the following error for the "link" assignment:

Unable to cast object of type 'Tridion.ContentManager.ContentManagement.Fields.SingleLineTextField' to type 'Tridion.ContentManager.ContentManagement.Fields.ComponentLinkField'.
   at Tridion.ContentManager.ContentManagement.Fields.ItemFieldsExtensions.IfExists[FIELDTYPE,OUT](ItemFields fields, String fieldName, Func`2 f)
   at ABC.NavigationXML.CreateProductsSolutionsXML.buildNavigation(XElement xRoot, Component component)
   at ABC.NavigationXML.CreateProductsSolutionsXML.buildNavigation(XElement xRoot, Component component)
   at ABC.NavigationXML.CreateProductsSolutionsXML.Transform(Engine engine, Package package)
   at Tridion.ContentManager.Templating.Assembly.AssemblyMediator.Transform(Engine engine, Template template, Package package)
   at Tridion.ContentManager.Templating.Assembly.CSharpSourceCodeMediator.RunTemplate(Engine engine, Package package, String templateUri, String className)
   at Tridion.Templating.CSharpTemplate.CSharpSourceTemplate.Transform(Engine __engine, Package __package)
   at    Tridion.ContentManager.Templating.Assembly.CSharpSourceCodeMediator.Transform(Engine engine, Template template, Package package)
   at Tridion.ContentManager.Templating.Engine.ExecuteTemplate(Template template, Package package)

Answer 1

I'd cover all bases and check if the schema field exists (what if someone renames this later or you decide to allow another schema that only has some of the currently covered fields)...

linkComponentFields.Component("linked_component") 

exists and then ensure that

linkComponentFields.Component("linked_component").Id 

has a value.

var xmlLink = string.Empty;
if(linkComponentFields.Component("linked_component") != null) {
    xmlLink = (linkComponentFields.Component("linked_component").Id != string.Empty ? linkComponentFields.Component("linked_component").Id | xmlLink);
  }

Answer 2

As Peter mentioned in his comments that your question is unclear. But if I consider your last comment which says its a "Object Reference" error, So I think it would be best to check the Objects Variables if those are NULL (a normal C# programming practice) before using them.

so please check objects like currentComponent, linkComponentFields , linkComponentFields.Component for a null condition before using them.

For example:

if(linkComponentFields.Component("linked_component") != null)
{
// code here
}