Steps to reproduce:

1. GO to Configuration -> Event correlation, click Create correlation
2. Add correlation Name
3. In New condition part choose 'New event host group'
4. Leave empty multiselect
5. Click Add link under New condition part
   Result:
   Error message:

   - Undefined index: groupids [ in correlation.php:236]
   - Incorrect value for field "groupid": cannot be empty.
   

   
   Expected:
   Error message without Undefined index:

   - Incorrect value for field "groupid": cannot be empty.