no possibility to reset check box "not null" in table editor

no possibility to reset check box "not null" in table editor

Hi,scenario:
1 open table editor  - ok
2  add new field  - ok
3 set "not null" check box - ok
4 jump to another field - ok
5 return to my new field - ok
6 try to reset "not null" check box  - ok
7 jump to another field - fiasco - "not null" check box return to ON value

dbforge studio for oracle 6.0.85