intermittent crash when creating new ladder node

From time to time, creating a node results in a fatal error. Syslogs attached containing incidents of each type. Once the issue starts, it occurs either when using inline entity form on an existing node (node/*/edit > add child step) or visiting /node/add/ladder. It does not always occur. Sometimes nodes save successfully. As I am writing this, they are not saving at all, but I've seen the issue come and go.

Steps to reproduce the problem:
1. log into drupalladder.org
2. create a new ladder node at /node/add/ladder
3. add at least the required title (at the moment the issue occurs even when only adding a title)
4. hit save
5. visit /admin/content and select to edit an existing node
6. add a child step; be sure to save the child and then save the main node

If the fatal error does not happen immediately in one/both of the above ways of saving, try a mix of the same ways of adding new nodes. It usually happens within a few attempts to save.

Example title on which it is currently failing:
Set up to communicate with other contributors and mentors