I had the same problem today, I terminated the NX Session and then deleted %USERPROFILE%\AppData\Local\Siemense\NX110\DialogMemory.dlx file.
When I started a new Session the problem disappeared.
Here's an FAQ on the same issue:
A similar issue was reported in PR 8969636. I was possible to enter a very small value for the balloon size, that NX interpreted as zero when the dialog was reopened (due to dialog memory) and hence the dialog failed to open. This issue was fixed in NX 12.0.2.