When trying to convert a private group to a public group, if you do not set a public link and tap on the top right checkmark, you'll see no errors and will go to the previous page.
Steps to reproduce:
You'll go back to the group settings with no errors (user may think that they successfully converted the group to a public one, but it's still a private group).
The empty link field should shake and the phone should vibrate (just like when you leave other fields empty).
Device info: Telegram Android 7.2.1, Huawei PRA-LA1 (Android 8)