can't pass instance variables from controller to view

task_controller_relevant parts

class TasksController < ApplicationController
  before_action :set_project
  before_action :set_task, except: [:index, :new, :create]

  def index
    @tasks = @project.tasks.all
    @completed = @tasks.completed
  end

  private

  def set_task
    @task = @project.tasks.find(params[:id])
  end

  def set_project
    @project = Project.find(params[:project_id])
  end

  def task_params
    params.require(:task).permit(:name, :deadline, :project_id)
  end
end

task_model

class Task < ApplicationRecord
  belongs_to :project
  has_many :comments, dependent: :destroy

  scope :completed, -> { where.not( completed: nil ) }
  scope :priority, -> { where( priority: true ) }
end

tasks - index.html.erb

<% @completed.each do |completed| %>
// this fails with undefined method `each' for nil:NilClass
hello <%= completed.name %>
<% end %>

<% @project.tasks.each do |completed| %>
// however this doesn't fail
hello <%= completed.name %>
<% end %>



<% @tasks.each do |task| %>
<p>
  <% if task.completed %>

// this goes alright

<% else %>
// this too
<% end %>
</p>
<% end %>