NoMethodError in Microposts#create - ruby-on-rails

Question

im having some trouble with my Ruby on Rails app. I'm new at this so i imagine the problem isn't too complicated
Basically I am trying to create a discussion board. the page /discussions has many discussions created by users who can make microposts to comment on them. When I click the post button for the micropost form however, i get an error that reads
NoMethodError in Microposts#create
Showing /home/nick/Documents/RailsProjects/buon/app/views/shared/_discussion_form.html.erb where line #1 raised:
undefined method `model_name' for NilClass:Class
any ideas? heres some of the code
micropost.rb
class Micropost < ActiveRecord::Base
attr_accessible :break_votes, :content, :not_votes
belongs_to :user
belongs_to :discussion
validates :content, presence: true, length: { maximum: 200 }
validates :user_id, presence: true
validates :discussion_id, presence: true
default_scope order: 'microposts.created_at DESC'
end
_discussions.html.erb
<% content_for :script do %>
<%= javascript_include_tag 'hover_content' %>
<% end %>
<li>
<div class = "intro-bar"><span class = "intro"><%=discussion.intro %></span></div>
<div class = "content-bar">
<span class = "content"><%= discussion.content %>
<div class = "vote-plus"><%= link_to "Stay Together", signup_path,class: "btn btn-large btn-primary" %></div>
<div class = "vote-neg"><%= link_to "Break Up", signup_path,class: "btn btn-large btn-primary" %></div>
</span>
</div>
<span class = "timestamp">
Posted <%= time_ago_in_words(discussion.created_at) %> ago.
</span>
</li>
<div class = "comments">
<% discussion.microposts.each do |micropost| %>
<li>
<div class = "post-comment"><%= micropost.content%></div>
</li>
<% end %>
</div>
<% if signed_in? %>
<div class = "row">
<aside class = "span4">
<section>
<%= render 'shared/micropost_form', :locals => {:discussion => discussion }%>
</section>
</aside>
</div>
<% end %>
micropost controller
class MicropostsController < ApplicationController
before_filter :signed_in_user, only: [:create, :destroy]
def index
end
def create
#micropost = current_user.microposts.build(params[:micropost])
if #micropost.save
flash[:success] = "Posted!"
redirect_to root_url
else
render 'static_pages/home'
end
end
def destroy
end
end
discussions_controller
class DiscussionsController < ApplicationController
before_filter :signed_in_user, only: [:index, :edit, :update]
def show
#user = User.find(params[:id])
#discussions = #user.discussion.paginate(page: params[:page])
#microposts = #user.micropost.paginate(page: params[:page])
end
def index
#discussions = Discussion.all
end
def create
#discussion = current_user.discussions.build(params[:discussion])
if #discussion.save
flash[:success] = "Discussion Started!"
redirect_to root_url
else
render 'static_pages/home'
end
end
def destroy
end
def edit
end
def update
end
def new
end
end
update: micropost_form
<% #micropost = Micropost.new %>
<% #micropost.discussion = nil %>
<%= form_for(#micropost) do |f| %>
<%= render 'shared/error_messages', object: f.object %>
<div class="field">
<%= f.text_area :content, placeholder: "Compose new micropost..." %>
</div>
<%= f.submit "Post", class: "btn btn-large btn-primary" %>
<% end %>