I have setup new CodeIgniter but i have getting this issue

Something is definitely out of whack: line 924 in the latest, rc.1, is in the middle of the stringify_attributes function, not slash_item. You didn't say which version you are running, but it doesn't look like the latest.

The line you bolded has a nullable return type declared, i.e. slash_item can return either a string or null.
This sounds more like a PHP version compatibility problem ... the git bash shell can have issues with PHP, from what I have seen.
Possible workaround: https://superuser.com/questions/1238410/...t-git-bash