NOT FOUND PAGE writes a project mx.exe cannot load (no error count emitted)

[mlehane1]

Summary

CREATE OR REPLACE NAVIGATION … NOT FOUND PAGE <Module>.<Page> is documented in Part IV (Navigation Profiles, Home Pages and Menus) as "an optional custom 404 page". It passes the syntax check, the apply step reports success and exits 0 — and the project can then no longer be loaded by mx.exe at all.

Repro

Two statements on a fresh mxcli new … --version 11.13.0 app:

CREATE OR REPLACE PAGE MyFirstModule.P404 (Title: '404', Layout: Atlas_Core.Atlas_Default) { };
CREATE OR REPLACE NAVIGATION Responsive
  HOME PAGE MyFirstModule.P404
  NOT FOUND PAGE MyFirstModule.P404;

then mx.exe check <app>.mpr.

Actual

Apply reports Navigation profile 'Responsive' updated., exit 0. Then:

ERROR: Mendix.Modeler.Storage.StorageLoadException: One or more invalid values were detected
  while loading the project
 - An error occurred when trying to set the 'NotFoundHomepage' property of a Navigation profile
 ---> System.ArgumentException: Object of type 'Mendix.Modeler.WebUI.Navigation.HomePage'
      cannot be converted to type '…NotFoundHomePage'.

There is no The app contains: N errors. line at all — the project fails to load rather than failing validation, so any tooling that diffs the error count sees nothing rather than a regression.

Isolated with a control

On a fresh copy, one statement apart:

step	result
baseline	0 errors
the same CREATE OR REPLACE NAVIGATION without the clause	0 errors (unchanged)
+ NOT FOUND PAGE	project will not load

Environment

Reproduced on mxcli 0.19.0 / Mendix 11.13.0, and independently on mxcli 0.20.0 / Mendix 11.11.0 — so it is neither version-specific nor fixed by 0.20.0's navigation rework.

Recovery

Headless: re-run CREATE OR REPLACE NAVIGATION without the clause and the project loads again.

Suggested fix

Write a NotFoundHomePage into that slot rather than a HomePage, or refuse the clause at apply time if the correct type cannot be produced. The current behaviour is a silent success that leaves an unopenable project.