a few more fixes

lib/flowerbox/result/failure_message.rb

@@ -43,13 +43,13 @@ module Flowerbox::Result
     end
 
     def first_local_stack
-      @first_local_stack ||= stack[1..-1].find do |line|
+      @first_local_stack ||= stack.find do |line|
         !system_files.any? { |file| line[%r{\(#{file}}] }
       end || stack[1] || ''
     end
 
     def exception?
-      stack[0][%r{^.+Error: }]
+      (stack[0] || '')[%r{^.+Error: }]
     end
   end
 end

lib/flowerbox/sprockets_handler.rb

@@ -2,6 +2,7 @@ require 'sprockets'
 require 'sprockets/engines'
 require 'forwardable'
 require 'sprockets-vendor_gems'
+require 'fileutils'
 
 module Flowerbox
   class SprocketsHandler
@@ -39,6 +40,8 @@ module Flowerbox
       return @environment if @environment
 
       @environment = Sprockets::Environment.new
+      FileUtils.rm_rf(Flowerbox.cache_dir)
+
       @environment.cache = Sprockets::Cache::FileStore.new(Flowerbox.cache_dir)
 
       self.class.gem_asset_paths.each { |path| append_path(path) }