First argument in form cannot contain nil or be empty

error messages partial

<% if object.errors.any? %>
<div id="error_explanation">
    <div class="alert alert-danger">
        The form contains <%= pluralize(object.errors.count, "error") %>
    </div>
    <ul>
    <% object.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
    <% end %>
    </ul>
    </div>
<% end %>

comment form

<%= form_for @comment, url: comments_path do |f| %>
 <%= render 'shared/error_messages', object: f.object %>
<%= f.hidden_field :user_id, value: current_user.id %>
<%= f.hidden_field :post_id, value: post.id %>
<%= f.text_area :content, size: "60x2", placeholder: "Comment on this post..." %>
 <%= f.submit "Comment" %>
<% end %>

post form

 <%= form_for [@user, @post] do |f| %>
 <%= render 'shared/error_messages', object: f.object %>
<%= f.text_area :content, size: "60x12", placeholder: "What do you want to say?" %>
<%= f.submit "Post" %>
 <% end %>

class CommentsController < ApplicationController
def index
    @comments = Comment.all
end

def new
    @comment = Comment.new
    @user = User.find(params[:user_id])
end

def create
    @user = current_user
    #@post = Post.find(params[:post_id])
    @comment = @user.comments.build(comment_params)

    if @comment.save
        flash[:success] = "Comment Posted!"
        redirect_back(fallback_location: root_path)
    else
        flash[:notice] = "Could not post comment"
        redirect_back(fallback_location: root_path)
    end
end


private

def comment_params
    params.require(:comment).permit(:content, :user_id, :post_id)
end
end

class PostsController < ApplicationController

def index
    @posts = Post.all
    @user = User.find(params[:user_id])
end

def new
    @post = Post.new
    @user = User.find(params[:user_id])
end

def create
    @post = current_user.posts.build(post_params)
    if @post.save
        flash[:success] = "Posted!"
        redirect_to user_path(current_user)
    else
        flash[:notice] = "Post could not be submitted"
        redirect_to users_path
    end
end

private

def post_params
    params.require(:post).permit(:content)
end
end