make default library nil

app/models/library.rb

@@ -175,7 +175,7 @@ class Library < ApplicationRecord
   end
 
   def self.default
-    Library.find_by(id: SiteSettings.default_library)
+    SiteSettings.default_library ? Library.find_by(id: SiteSettings.default_library) : Library.first
   end
 
   def default?

app/models/site_settings.rb

@@ -20,7 +20,7 @@ class SiteSettings < RailsSettings::Base
   field :default_viewer_role, type: :string, default: "member"
   field :approve_signups, type: :boolean, default: false
   field :theme, type: :string, default: "default"
-  field :default_library, type: :integer, default: Library.first&.id
+  field :default_library, type: :integer, default: nil
 
   validates :model_ignored_files, regex_array: {strict: true}