check if user has signed in

app/views/jobs/show.html.erb

@@ -32,7 +32,7 @@
 
 <div class="job_detail_button">
   <%= link_to "Favorite", "#", class: "btn btn-primary float-right" %>
-  <% if UserJob.find_by(user_id: current_user.id, job_id: @job.id) %>
+  <% if user_signed_in? && UserJob.find_by(user_id: current_user.id, job_id: @job.id) %>
     <button type="button" class="btn btn-primary float-right mx-3" disabled>Apply</button>
   <% else %>
     <%= link_to "Apply", apply_path(job_id: @job.id), class: "btn btn-primary float-right mx-3" %>