Editor crashes when importing 4.5.1 project to 4.6

The user converted a copy of their 4.5.1 project to 4.6. Upon opening that project, the editor crashes. Deleting certain widgets allows the user to open the project, but opening any map results in an exception:
Fatal error: [Link Removed] [Line: 11]
Cast of nullptr to Blueprint failed

Crash Reporter link: [Link Removed]

This second user also seems to have the same problem; their crash reports are near-identical.
https://answers.unrealengine.com/questions/143149/crash-after-migrating-to-46-on-open-blueprint.html#answer-144638
[Link Removed]

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