Add fallback for last service date and introduce 'unknown' label

app/views/vehicles/_details.html.erb

@@ -54,7 +54,7 @@
   <% mileage = last.custom_values.find_by(custom_field_id: CustomField.find_by_name("Mileage").id) %>
   <% if mileage %>
     <p>
-      <b><%=t(:label_last_service) %></b> <%= last.closed_on.to_date %>
+      <b><%=t(:label_last_service) %></b> <%= last&.closed_on&.to_date || l(:label_unknown) %>
       at <%= mileage.value %>
       <%=t(:label_miles) %>
     </p>