MVVM ViewBinding in UserWidget causes cook failure and permanently corrupts the widget

jasperlnz
Tue, 09 Dec 2025 11:00:16 +0000
Issues and Bug Reporting

Summary

Creating MVVM ViewBindings inside a Modal Dialog Variant (especially Button event bindings for the Pop-up Dialog Device) consistently causes a cook failure when launching in-editor.

After the cook failure, UEFN crashes when reloading the widget, and the widget becomes permanently corrupted and cannot be reopened nor reloaded.

Please select what you are reporting on:

Unreal Editor for Fortnite

What Type of Bug are you experiencing?

Assets

Steps to Reproduce

Create a new Modal Dialog Variant Widget Blueprint in UEFN.

Start adding ViewBindings to buttons with the CreativeModalDialogViewmodels and add Events such as OnClicked/OnHovered/OnUnhovered (Button Events were the most common trigger).

At some point during binding, deleting & keeping a few bindings empty, the following message appears in the output log:
Ensure condition failed: false – MVVMBlueprintViewEvent… was created in invalid Outer /Script/CoreUObject.Package

Save the widget and attempt to launch in (“Start Session”).

The launch fails with a Cook Failure.

Close and restart UEFN.

Try to open or reload the affected widget.

UEFN crashes immediately. The widget is now permanently corrupted:
• cannot open
• cannot reload
• cannot validate
• duplicating still crashes

Expected Result

The Widget Blueprint with ViewModel/ViewBindings should launch normally after cooking, and should reopen without crashing after restarting UEFN.

Observed Result

• Cook Failure during launch
• After restart, opening or reloading the UserWidget instantly crashes UEFN
• The widget becomes unrecoverable and breaks the project state
• Crash reporter shows:
EXCEPTION_ACCESS_VIOLATION
and
Ensure condition failed: false – MVVMBlueprintViewEvent_0 … created in invalid Outer /Script/CoreUObject.Package

Platform(s)

PC – Windows 11
UEFN Latest Public Version

Additional Notes

• The corruption only occurs after ViewBindings are created.
• Duplicating the widget before adding bindings works fine.
• After adding bindings and triggering the MVVM error message, the next cook attempt breaks the asset permanently.
• Rebuilding the widget from scratch reproduces the same failure as soon as MVVM bindings are added (And the error message appears)

7 posts - 4 participants