A PHP Error Severity: Notice Message: session_write_close(): Skipping numeric key 0

The keys in the $ _SESSION associative array are subject to the same limitations as regular variable names in PHP,
ie they can not start with a number and must start with a letter or underscore.

For more details see the section on variables in the PHP manual.

Or you did not follow the upgrade right.

I have followed the upgrade instructions. This error goes away if I write Session_write_close(); in the beginning of that controller class.
I just wanted to confirm is this a right way to do.