Ketenstandaard unable to create new message spec version

Problem / goal

As we can see on Discord, Ketenstandaard experienced loss of functionality in v4, stopping them from creating a new version of their ItemProductMessage message specification (v007).

Like #1529 (closed) and #1530 (closed), the problem is a missing button:

In the edit message model version screen, the new message button has disappeared. This button was the only way for Ketenstandaard to create a new message model version for an existing message model spec, using wizard-like functionalities, without using the actual wizard (since that involves creating an entirely new specification too).

(This goes to show that we need to work on #1463 )

What is the expected outcome?

Quick fix: bring back the New Message button in V4