Under some circumstances, clicking on up and down buttons to re-order events inside a workflow produces a fatal error.

Steps to reproduce:

1. Set up workflow with an "approval" event followed by a "wait until date" event;
2. Set as approvers some user which you will later remove;
3. Save workflow;
4. Remove approver user;
5. Edit workflow again. At this stage, a warning will be displayed:

The workflow could not be stored.

The following information is either missing or invalid:

(1) Event / Approve: 
Some of passed user IDs are not valid, must be IDs of existing users only.
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6. Ignore it, and try to move up the "wait until date" event. A fatal error will be generated:

Fatal error: The web server did not finish its request