Getting undefined method 'map' for Nil Class (coming from Kind.all on the select_options method)

_form.html.erb

<%= form_for(@contact, html: { class: "basic-grey" }) do |f| %>
  <% if @contact.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@contact.errors.count, "error") %> prohibited this contact from being saved:</h2>

      <ul>
      <% @contact.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <h1>Contacts</h1>

  <div class="field">
    <%= f.label :name %><br>
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :email %><br>
    <%= f.text_field :email %>
  </div>
  <div class="field">
    <%= f.label :kind_id %><br>
    <%= collection_select(:contact, :kind_id, @kind_options_for_select, :id, :description) %>
  </div>


  <%= f.fields_for :address do |address_fields| %>
    <div class="field">
      <%= f.label :street %><br>
      <%= address_fields.text_field :street %>
    </div>
    <div class="field">
      <%= f.label :city %><br>
      <%= address_fields.text_field :city %>
    </div>
    <div class="field">
      <%= f.label :state %><br>
      <%= address_fields.text_field :state %>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :rmk %><br>
    <%= f.text_area :rmk %>
  </div>
  <div class="field">
    <%= f.submit "Enviar", class: "button" %>
    <%= link_to 'Back', contacts_path, class: "button"%>
  </div>
<% end %>

contact.rb

class Contact < ApplicationRecord
  belongs_to :kind
  has_one :address

  accepts_nested_attributes_for :address
end

contacts_controller.rb

class ContactsController < ApplicationController
  before_action :set_contact, only: [:show, :edit, :update, :destroy]

  # GET /contacts
  # GET /contacts.json
  def index
    @contacts = Contact.all
    @meu_nome = "Crova"
  end

  # GET /contacts/1
  # GET /contacts/1.json
  def show
  end

  # GET /contacts/new
  def new
    @contact = Contact.new
    @contact.build_address

    options_for_select
  end

  # GET /contacts/1/edit
  def edit
    options_for_select
  end

  # POST /contacts
  # POST /contacts.json
  def create
    @contact = Contact.new(contact_params)
    @contact.build_address

    respond_to do |format|
      if @contact.save
        format.html { redirect_to @contact, notice: 'Contact was successfully created.' }
        format.json { render :show, status: :created, location: @contact }
      else
        format.html { render :new }
        format.json { render json: @contact.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /contacts/1
  # PATCH/PUT /contacts/1.json
  def update
    respond_to do |format|
      if @contact.update(contact_params)
        format.html { redirect_to @contact, notice: 'Contact was successfully updated.' }
        format.json { render :show, status: :ok, location: @contact }
      else
        format.html { render :edit }
        format.json { render json: @contact.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /contacts/1
  # DELETE /contacts/1.json
  def destroy
    @contact.destroy
    respond_to do |format|
      format.html { redirect_to contacts_url, notice: 'Contact was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

  private

    def options_for_select
      @kind_options_for_select = Kind.all
    end


    # Use callbacks to share common setup or constraints between actions.
    def set_contact
      @contact = Contact.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def contact_params
      params.require(:contact).permit(:name, :email, :kind_id, :rmk, address_attributes: [:street, :city, :state])
    end
end
 

you're doing it for new and edit actions which render the form initially, but you aren't doing it on create and update actions which both re-render the form in case of any input error

Ohh, I see, rookie mistake

Cheers mate