Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

<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:
[015942DA]

Status: symbol_unknown

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.