"Checks" for attributes can be annotated on the model in order to express consistency rules. On the GUI side, it is not always easy to point out inconsistencies at an early stage. A meaningful message should then appear at the latest when saving. This is already happening. However, the message should be better presented visually as a failed check. Currently, a red box with the heading "Internal error" is displayed, which suggests a bug/runtime error and not an invalid input by the user.