Fix navigation issues when primary Geo node has been destroyed

app/controllers/admin/application_controller.rb

@@ -12,6 +12,7 @@ class Admin::ApplicationController < ApplicationController
 
   def display_geo_information
     return unless Gitlab::Geo.secondary?
+    return unless Gitlab::Geo.primary_node.present?
 
     primary_node = view_context.link_to('primary node', Gitlab::Geo.primary_node.url)
     flash.now[:notice] = "You are on a secondary (read-only) Geo node. If you want to make any changes, you must visit the #{primary_node}.".html_safe

app/views/layouts/nav/_dashboard.html.haml

@@ -60,7 +60,7 @@
       = link_to sherlock_transactions_path, class: 'admin-icon', title: 'Sherlock Transactions',
         data: {toggle: 'tooltip', placement: 'bottom', container: 'body'} do
         = icon('tachometer fw')
-  - if Gitlab::Geo.secondary?
+  - if Gitlab::Geo.secondary? && Gitlab::Geo.primary_node.present?
     %li
       = link_to Gitlab::Geo.primary_node.url, title: 'Go to primary node', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'} do
         = icon('globe fw')