What's wrong with my Ruby on Rails code?

So I've been trying to add some AJAX to my home page with rjs and I get this javascript error in a popup window:

RJS error:

TypeError: Cannot call method 'getElementsByTagName' of null

Below is all of the relevant code.

Here's my code in app/views/microposts/create.rjs:

page.insert_html :bottom, :feed_items, :partial => 'shared/feed_item', :object => @micropost
page.replace_html :user_info, pluralize(current_user.microposts.count, "micropost")
page[:micropost_form].reset
page.replace_html :notice, flash[:notice]
flash.discard

Here's app/views/pages/home.rb:

<% if signed_in? %>
  <table class="front" summary="For signed-in users">
    <tr>
      <td class="main">
      <h1 class="micropost">What's happening?</h1>
    <%= render 'shared/micropost_form' %>
    <%= render 'shared/feed' %>
  </td>
  <td class="sidebar round">
    <%= render 'shared/user_info' %>
    <%= render 'shared/stats' %>
  </td>
 </tr>
</table>
<% else %>
<h1>Sample App</h1>

<p>
  This is the home page for the
  <a href="http://railstutorial.org/">Ruby on Rails Tutorial</a>
  sample application.
</p>

<%= link_to "Sign up now!", signup_path, :class => "signup_button round" %>
<% end %>

Here's my feed partial app/views/shared/_feed.html.erb:

<% unless @feed_items.empty? %>
  <table id="feed_items" class="microposts" summary="User microposts">
    <%= render :partial => 'shared/feed_item', :collection => @feed_items %>
  </table>
  <%= will_paginate @feed_items %>
<% end %>

Here's my feed item partial app/views/shared/_feed_item.html.erb

 <tr id = "feed_item">
  <td class="gravatar">
    <%= link_to gravatar_for(feed_item.user), feed_item.user %>
  </td>
 <td class="micropost">
    <span class="user">
      <%= link_to feed_item.user.name, feed_item.user %>
    </span>
    <span class="content"><%= feed_item.content %></span>
    <span class="timestamp">
      Posted <%= time_ago_in_words(feed_item.created_at) %> ago.
    </span>
  </td>
  <% if current_user?(feed_item.user) %>
  <td>
    <%= link_to "delete", feed_item, :method => :delete,
                                     :confirm => "You sure?",
                                     :title => feed_item.content %>
  </td>
  <% end %>
</tr>

And lastly here is the relevant part of my microposts controller:

class MicropostsController < ApplicationController
  before_filter :authenticate, :only => [:create, :destroy]
  before_filter :authorized_user, :only => :destroy

  def create
    @micropost  = current_user.microposts.build(params[:micropost])
    respond_to do |format|
      if @micropost.save
        flash[:success] = "Micropost created!"
        format.html { redirect_to root_path }
        format.js   
      else
        @feed_items = []
        render 'pages/home'
      end  
    end
  end