Rails AJAX link_to not refreshed controller action - ruby-on-rails

Question

In my Rails 5 app, Doctor/Admin have to create a Caregiver with linked Patient (new object CaregiverPatient represents that). During this process, inside the registrants_controller#new, Doctor/Admin search for a Patient (which is done by Ransack gem) and from the result he/she push Add Patient button to add Patient to initialized CaregiverPatient object from caregiver_patient#new and display #patient.full_name inside the form.
In order not to lose already filled form fields, everything must be done asynchronously. I've tried to do so with AJAX, below my code:
registrant_controller.rb
def new
#registrant = Registrant.new
#patient = Registrant.find(session[:patient_id_to_add_caregiver]) if session[:patient_id_to_add_caregiver]
end
registrants/_new_form.html.erb
<%= form_for #registrant do |f| %>
<%= f.fields_for :registration do |registration| %>
<% if #patient %>
<div id="add-patient"></div>
<% else %>
<%= required_text_field_group registration, #registrant, :registrant_id, "Patient Added", {}, { disabled: true } %>
<% end %>
<% end %>
# some other logic (...)
# patient search result
<% #registrants do |registrant| %>
<%= link_to 'Add Patient', new_registrant_link_path(registrant), remote: true %>
<% end %>
Add Patient button triggers caregiver_patients#new action:
#caregiver_patient_controller.rb
class CaregiverPatientsController < ApplicationController
# new_registrant_link_path
def new
session[:patient_id_to_add_caregiver] = params[:registrant_id].to_i
respond_to do |format|
format.html { redirect_to request.referer }
format.js { render action: 'patient_to_caregiver', notice: 'Patient Added' }
end
end
end
AJAX request is made:
#views/caregiver_patients/patient_to_caregiver.js.erb
$("#add-patient").html("<%= escape_javascript(render :partial => 'registrants/add_patient') %>");
views/registrants/_add_patient.html.erb
Patient Added: <%= #patient.full_name %>
But surprisingly gets an error:
ActionView::Template::Error (undefined method `full_name' for nil:NilClass):
1: Patient Added: <%= #patient.full_name %>
Which probably means that registrants_controller#new was not refreshed under the hood somehow. What have I missed?