<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 5.7.7

Error message:
Control%20%27MainMessage%27%20has%20no%20parent%20window%0D%0A%0D%0AStack%20trace%3A%0D%0A%2800216582%29%20Vcl%3A%3AControls%3A%3ATControl%3A%3AGetDeviceContext%0D%0A%28002146F3%29%20Vcl%3A%3AControls%3A%3ATControlCanvas%3A%3ACreateHandle%0D%0A%28001BFB05%29%20Vcl%3A%3AGraphics%3A%3ATCanvas%3A%3ARequiredState%0D%0A%280017FFA3%29%20Vcl%3A%3AStdctrls%3A%3ATCustomLabel%3A%3AAdjustBounds%0D%0A%28001801D4%29%20Vcl%3A%3AStdctrls%3A%3ATCustomLabel%3A%3ACMTextChanged%0D%0A%2800217700%29%20Vcl%3A%3AControls%3A%3ATControl%3A%3APerform%0D%0A%2800215541%29%20Vcl%3A%3AControls%3A%3ATControl%3A%3ASetTextBuf%0D%0A%2800215FAB%29%20Vcl%3A%3AControls%3A%3ATControl%3A%3ASetName%0D%0A%2800768ED5%29%20TMessageForm%3A%3ACreate%0D%0A%280076AFD5%29%20CreateMoreMessageDialog%0D%0A%280009F2DA%29%20CreateMessageDialogEx%0D%0A%280009FA45%29%20MoreMessageDialog%0D%0A%28000A1160%29%20ExceptionMessageDialog%0D%0A%280008378A%29%20ShowExtendedExceptionEx%0D%0A%280001AB28%29%20TCustomScpExplorerForm%3A%3AShowExtendedException%0D%0A%2800077BDD%29%20TTerminalManager%3A%3ATerminalShowExtendedException%0D%0A%280070FC38%29%20TTerminal%3A%3AShowExtendedException%0D%0A%28000A9A85%29%20Execute%0D%0A%2800002462%29%20wWinMain%0D%0A%28007DEA01%29%20C4667_0%0D%0A%2800038900%29%20ntdll.dll%0D%0A%28000388D0%29%20ntdll.dll

Status: not_matched

The problem happens rarely. I cannot reproduce it.