NoMethodError: undefined method `downcase' for nil:NilClass (NoMethodError)

    user = User.find_by(email: params[:session][:email].downcase)
                                                       ^^^^^^^^^
  app/controllers/sessions_controller.rb:50:in `standard_authentication'
    user = User.find_by(email: params[:session][:email].downcase)
  app/controllers/sessions_controller.rb:12:in `create'
    standard_authentication
...
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