form_for NoMethodError in Entities#new

Tried to create a form for assosiated model in rails but form_for giving error " NoMethodError in Entities#new " " undefined method `entities' for nil:NilClass"

Code for model

<!--Schema model -->

class Schema < ApplicationRecord
  has_many :entities, dependent: :destroy

  validates :name, presence: true, uniqueness: true
  validates :schemaId, presence: true, uniqueness: true
end

<!--Entity Model -->

class Entity < ApplicationRecord
  belongs_to :schema
end

<!-- Entity Controller -->
class EntitiesController < ApplicationController

  def index
    @schema = Schema.find_by(:id => params[:id])
    @entities = @schema.entities
  end


  def new
      @schema = Schema.find_by(:id => params[:id])
      @entity = Entity.new
  end

  def create
    @schema = Schema.find_by(:id => params[:id])
    @entity = @schema.entities.new(entity_params)
    @entity.save!
  end

  private

    def entity_params
       params.require(:entity).permit(:clientId, :name, :description, :mnemonic)
   end
end



<!-- Entity_new_form-->
<%= form_for [@schema, @schema.entities.build] do |f| %>
  <p>
    <%= f.label :clientId %><br/>
    <%= f.text_field :clientId %>
  </p>
  <p>
    <%= f.label :name %><br/>
    <%= f.text_area :name %>
  </p>

  <p>
    <%= f.label :description %><br/>
    <%= f.text_area :description %>
  </p>
  <p>
    <%= f.label :mnemonic %><br/>
    <%= f.text_area :mnemonic %>
  </p>
  <p>
    <%= f.submit 'Submit' %>
  </p>
<% end %>

This is giving error here entity is associated with Schema... I have to take input from user for new entity but this form is giving me error.