one more addition improvement in error_report

added check for `project_podfile` variable and only if it's not nil wi
will check `exist?`

lib/cocoapods/command/error_report.rb

@@ -59,7 +59,7 @@ EOS
         private
 
         def markdown_podfile
-          return '' unless Config.instance.project_podfile.exist?
+          return '' unless Config.instance.project_podfile && Config.instance.project_podfile.exist?
 <<-EOS
 
 ### Podfile