DB Upgrade to 19c using DBUA

ERROR:
======
Error in preupgrade tool execution. ERROR - Unable to run the preupgrade due to:ERROR - Unable to run preupgrade due to:
ORA-06512: at "SYS.UTL_FILE", line 106
ORA-06512: at "SYS.UTL_FILE", line 746
ORA-06512: at "SYS.DBMS_PREUP", line 3352
ORA-06512: at "SYS.DBMS_PREUP", line 10500
ORA-06512: at line 8

declare
*
ERROR at line 1:
ORA-29284: file read error
ORA-06512: at line 56

SOLUTION:
==========
1) unset the env variable ORA_NLS10
unset ORA_NLS10

Verify:
echo $ORA_NLS10

2) Re-run DBUA upgrade.