Fix #146 warnings for xvfb

Author: route

Rakefile

@@ -3,6 +3,8 @@
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 
-RSpec::Core::RakeTask.new("test")
+RSpec::Core::RakeTask.new("test") do |t|
+  t.ruby_opts="-w"
+end
 
 task default: :test

lib/ferrum/browser/process.rb

@@ -64,6 +64,7 @@ def initialize(options)
           return
         end
 
+        @xvfb = nil
         @logger = options[:logger]
         @process_timeout = options.fetch(:process_timeout, PROCESS_TIMEOUT)