Plugin fails to build: undefined method `+' for nil:NilClass

I’m currently using an i18n plugin called jekyll-multiple-languages-plugin for my Jekyll website hosted by GitLab Pages. I’m using the modified script klaasnotfound/jekyll-multiple-languages-plugin.rb for the plugin. However, when I do build the site, it gives me the error: undefined method '+' for nil:NilClass (NoMethodError).

Here is the full traceback:

Configuration file: C:/Users/Pradana/umarsaid/_config.yml
            Source: C:/Users/Pradana/umarsaid
       Destination: C:/Users/Pradana/umarsaid/_site
 Incremental build: disabled. Enable with --incremental
C:/Users/Pradana/umarsaid/_plugins/jekyll-multiple-languages-plugin/jekyll-multiple-languages-plugin.rb:154:in `block in process': undefined method `+' for nil:NilClass (NoMethodError)
	from C:/Users/Pradana/umarsaid/_plugins/jekyll-multiple-languages-plugin/jekyll-multiple-languages-plugin.rb:150:in `each'
	from C:/Users/Pradana/umarsaid/_plugins/jekyll-multiple-languages-plugin/jekyll-multiple-languages-plugin.rb:150:in `process'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/jekyll-3.5.0/lib/jekyll/command.rb:26:in `process_site'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/jekyll-3.5.0/lib/jekyll/commands/build.rb:63:in `build'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/jekyll-3.5.0/lib/jekyll/commands/build.rb:34:in `process'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/jekyll-3.5.0/lib/jekyll/commands/build.rb:16:in `block (2 levels) in init_with_program'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
	from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/jekyll-3.5.0/exe/jekyll:13:in `<top (required)>'
	from C:/Ruby24-x64/bin/jekyll:22:in `load'
	from C:/Ruby24-x64/bin/jekyll:22:in `<main>'

Here is class Site of klaasnotfound/jekyll-multiple-languages-plugin.rb where the issue is being traced back to.

class Site

    attr_accessor :parsed_translations   # Hash that stores parsed translations read from YAML files.

    alias :process_org :process

    # process
    # Reads Jekyll and plugin configuration parameters set on _config.yml, sets
    # main parameters and processes the website for each language.
    def process
      # Check if plugin settings are set, if not, set a default or quit.
      self.parsed_translations ||= {}

      self.config['exclude_from_localizations'] ||= []

      if ( !self.config['languages']         or
            self.config['languages'].empty?  or
          puts 'You must provide at least one language using the "languages" setting on your _config.yml.'


      # Variables

      # Original Jekyll configurations
      baseurl_org                 = self.config[ 'baseurl' ] # Baseurl set on _config.yml
      dest_org                    = self.dest                # Destination folder where the website is generated

      # Site building only variables
      languages                   = self.config['languages'] # List of languages set on _config.yml

      # Site wide plugin configurations
      self.config['default_lang'] = languages.first          # Default language (first language of array set on _config.yml)
      self.config[        'lang'] = languages.first          # Current language being processed
      self.config['baseurl_root'] = baseurl_org              # Baseurl of website root (without the appended language code)
      self.config['translations'] = self.parsed_translations # Hash that stores parsed translations read from YAML files. Exposes this hash to Liquid.

      # Build the website for all languages

      # Remove .htaccess file from included files, so it wont show up on translations folders.
      self.include -= [".htaccess"]

      languages.each do |lang|

        # Language specific config/variables
        @dest                  = dest_org    + "/" + lang
        self.config['baseurl'] = baseurl_org + "/" + lang
        self.config['lang']    =                     lang

        puts "Building site for language: \"#{self.config['lang']}\" to: #{self.dest}"


      # Revert to initial Jekyll configurations (necessary for regeneration)
      self.config[ 'baseurl' ] = baseurl_org  # Baseurl set on _config.yml
      @dest                    = dest_org     # Destination folder where the website is generated

      puts 'Build complete'

    if <"3.0.0")
      alias :read_posts_org :read_posts

      # read_posts
      def read_posts(dir)
        translate_posts = !self.config['exclude_from_localizations'].include?("_posts")

        if dir == '' && translate_posts