Steps to reproduce:

1. Open widget (Top Items (v.7.2+), Item History, Top Hosts)
2. Go to columns Add section
3. Enter Name and any Item name (Non-existing also viable)
4. Press and Hold ENTER button

Result:

Multiply columns are added.
Add Window is reopened with prefilled data and validation is missing for second add attempt 

Expected:
One column is added.

If new window is opened it is clear of previous data and in case of empty fields -proper error message is displayed

Example: