Problem/Motivation
Check the issue title ;)
Proposed resolution
Override the preDelete() method in ConfigEntityBundleBase and throw an exception when there are existing content entities using the bundle that is about to be deleted.
Remaining tasks
Review.
User interface changes
Nope.
API changes
Deleting config entities that act as bundles for content entities is now prevented on the API level when there is existing data for those bundles.