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

Canceled a lengthy FTP transfer.

<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 5.7

Error message:
Control 'MainMessage' has no parent window

Stack trace:
[002A66CE] Vcl::Controls::TControl::GetDeviceContext
[002A483F] Vcl::Controls::TControlCanvas::CreateHandle
[0024FC51] Vcl::Graphics::TCanvas::RequiredState
[002100EF] Vcl::Stdctrls::TCustomLabel::AdjustBounds
[00210320] Vcl::Stdctrls::TCustomLabel::CMTextChanged
[002A784C] Vcl::Controls::TControl::Perform
[002A568D] Vcl::Controls::TControl::SetTextBuf
[002A60F7] Vcl::Controls::TControl::SetName
[007F4F29] TMessageForm::Create
[007F7029] CreateMoreMessageDialog
[0012F9FE] CreateMessageDialogEx
[00130169] MoreMessageDialog
[00131884] ExceptionMessageDialog
[00114252] ShowExtendedExceptionEx
[001139A0] ShowExtendedException
[00139BD1] Execute
[00093456] wWinMain
[0086E735] __wstartup
[77D89F70] ntdll.dll
[77D89F40] ntdll.dll