How can we reproduce it (please see attached gif)
1 Create new Project
2 Go to Project Browser and do RightMouseClick->Create->Material and set 260+symbols name (e.g. 1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890a1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890a12345678123456789012345678901234567890123456789011111111111111111111)
3 Click somewhere on the screen or press Enter
4 Pop up will appear - press Cancel
5 Click on empty space near the last asset in Project Browser

Actual result: an invisible object is created
Expected result: there should not be any invisible objects

Note: after Editor restart - the invisible object disappears
Checked only on Windows 10:
- Reproduces on 2020.2.0b5 and 2021.1.0a1
- On 2019.4.2f1 the behavior is slightly different - the same pop up (on step 4) appears two times in a row and after pressing Cancel on both of them the Invisible object doesn't appear - so, I am not sure if we should call this a regression or not (2 popups are the problem, but Invisible object doesn't appear) - feel free to change that field