Information box content lost when multiple users update the macro parameters at the same time

Follow these steps to reproduce:

• Edit the same page with two users, Alice and Bob
• Alice inserts an information box and types some content inside
• Alice opens the Macro Edit modal for the inserted information box to update the CSS class parameter, without closing it
• At the same time Bob also opens the Macro Edit modal for the same information box and updates the CSS class parameter
• Bob submits the changes just before Alice

Expected: Alice's value for CSS class parameter should overwrite Bob's value, but the information box content should remain unchanged

Actual: both users get a rendering error that the information box content is missing

The reason is this: when Alice submits her changes, instead of updating the existing information box macro, it actually inserts a new macro (with no content) that overwrites the previous one which has been updated in the meantime by Bob.