Visual Editor does not show error message for empty required Text field

When a required Text field in the Visual Editor becomes empty, the error message below the field is not shown.

Steps to reproduce:

1. In the HAP, open the Christmas Tree document in the Visual Editor
2. Make the 'Quote' field empty

Expected: a message is shown below the field saying "Quote is a required field"

Actual: no message is shown. The JavaScript console shows the following warning:

mdInput messages show animation called on invalid messages element:
[md-input-container.qa-field.ng-scope.md-input-has-placeholder.has-focused-field.md-input-focused.ng-…]