fix: Load navbar correctly if there is no current course

Author: yammesicka

lms/templates/navbar.html

@@ -84,7 +84,7 @@
         {% endif -%}
       </ul>
       <ul class="navbar-nav ms-auto">
-        {%- if current_user.role.is_manager %}
+        {%- if current_user.role.is_manager and current_user.last_course_viewed %}
         <li class="nav-item">
           <a href="{{ url_for('status', course_id=current_user.last_course_viewed.id) }}" class="nav-link">
             <i class="fa fa-check-square-o" aria-hidden="true"></i>