app/views/timeline_events/_form.html.erb
<%= form_with(model: timeline_event, local: true) do |form| %>
<% if timeline_event.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(timeline_event.errors.count, "error") %> prohibited this timeline_event from being saved:</h2>
<ul>
<% timeline_event.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="actions">
<%= form.submit %>
</div>
<% end %>