the Country item create bug in Fields builder

I installed Dolphin 7.0.4.   When I create a new Country item in Fields builder, I name it TCountry, but I couldn't save it. there is no reaction. and since then,  I couldn't create any items. I reinstalled Dolphin 7.0.4. and the same problem happend again.  I never saw this kind of bugs even in 7.0.2 or 7.0.3.  I saved the picture in attach file. please check it out.

This problem is already resolved.