ORA-12838: cannot read/modify an object after modifying it in parallel

User try to open PO period, users got the error ORA-12838: cannot read/modify an object after modifying it
in parallel.

system error : Bug 28039754 : OPEN PERIOD ERROR ORA-12838

PROBLEM STATEMENT
=================
Open period first period of 2018 Error :
gl_summary_account_types_pkg.update_account_types() 
SHRD0114: Executing Pro*C statement Update acct type...

SHRD0043: ERROR: ORA-12838: cannot read/modify an object after modifying it
in parallel

Solution:  

1. make sure gl.gl_code_combinations is not parallel.

2.  Make sure your GL period is open.

3. finally open the PO period.