No DataSources assigned to application component &1 yet

What causes this issue?

You are deleting an application component in the application component
hierarchy in the source system. However, DataSources are still assigned
to this application component hierarchy.
When you save the hierarchy, you can find the respective DataSources in
the additional maintenance as well as in the connected BW systems under
the node NODESNOTCONNECTED.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.