New Create world UI always presents a same world seed

New Create world UI always presents a same random world seed:3257840388504953787.

Steps to reproduce:

1.open the new world creating UI.

2.remember the random seed in the "advanced"-"World seed" textbox.

3.close the UI.

4.open the new world creating UI again and pay attention to the new random world seed.

Expected results:

New Create world UI will show different random seeds while opened.

Observed result:

New Create world UI always presents a same random world seed:3257840388504953787.