【Rails】Validation failed: 〜 must existというエラーの対処法

ActiveRecord::RecordInvalid (Validation failed: Lost must exist):

class Matter < ApplicationRecord
  belongs_to :user
  belongs_to :lost, optional: true #ここを追加
end

class Lost < ApplicationRecord
  has_many :matters, dependent: :destroy
end

class Matter < ApplicationRecord
  belongs_to :lost, optional: true
end

belongs_to :lost, optional: true

class MattersController < ApplicationController
  def q1_after
    @matter=Matter.new(user_id:current_user.id, is_solved:false)
    @lesson=Lesson.find_by(name:params[:lesson])
    category=params[:category]
    if @matter.save #lesson_idが空の状態
      redirect_to("/q2/#{@matter.id}/#{@lesson.id}/#{category}")
    else
    flash[:notice]="やり直してください"
    redirect_to("/q1")
    end
  end
end