When you remove a credential that is still being used as a credential repository entry (for example in a connection definition) there is no check.
If you check the properties of the entry that had the removed credential in its definition the pop-up "Unable to find the credential entry, the setting will be lost!" appears.
Prior to delete your credential entry, just select it in the Navigation Pane. In the Dashboard, you will see a Referenced by tab. This will allow you to see in which sessions your credential entry is used.
Although our various support queues will be monitored for emergencies, Devolutions' offices will be closed on Monday, May 20, 2019.
I will check to add a check but it will not be able to detect the link if you don't have access to the entry.