Steps to reproduce:

1. Create an issue type.
2. Copy any existing workflow.
3. Go to an existing workflow scheme and add the add the inactive workflow from Step 2.
4. Associate the new issue type to the workflow above in Step 3, and publish the workflow scheme.
5. Delete the issue type created in Step1.
6. Check workflow list and there will be one active workflow without any associated scheme.

Workaround:

1. Create a dummy issue type.
2. Assign the issue type to the affected workflow in any workflow scheme, and publish it.
3. Remove the association first and the issue type can be deleted now without leaving the workflow as active.