Control '' has no parent window

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.7.1

Error message:
Control '' has no parent window

Stack trace:
[005242DA]

Status: symbol_unknown

The problem happens rarely. I cannot reproduce it.