How to make Project Manager information mandatory for Projects?

Summary:

How to make Project Manager as a mandatory information for Projects for entire duration of project.

Content (required):

We have a requirement to make Project Manager as a mandatory information for Projects since it will be required for routing various project-based requisition and invoice approval workflows.

Currently, we have set Project Manager as a required Quick Entry on the Project Template. However, a user can end-date the Project Manager at any point and proceed to save the changes without adding a new Project Manager to the project.

How can we restrict this behavior? Else, is there a way to display a warning message to the users in this scenario?