Clean-Up not possible (see longtext)

What causes this issue?

The Project contains only one invalid Entity.

System Response

After the Clean-Up the project will be empty, which cause a dump in the
Service Builder.

How to fix this error?

First import a valid Entity and select it. Try the Clean-Up again.
If the project is no longer required delete the project.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.