<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 5.15.1

Error message:
Invalid access to memory.

Stack trace:
(0027B623) Vcl::Controls::TWinControl::HandleNeeded
(0008402F) ntdll.dll
(00071931) ntdll.dll.KiUserExceptionDispatcher
(0048FEBB) Tb2extitems::TTBEditItemViewer::EditLoop
(004900B2) Tb2extitems::TTBEditItemViewer::DoExecute
(0049F75A) Tb2item::TTBModalHandler::Loop
(0049E8EB) Tb2item::TTBView::EnterToolbarLoop
(004AC396) Tb2toolbar::TTBToolbarView::EnterToolbarLoop
(00274308) Vcl::Controls::TControl::DoMouseDown
(00274357) Vcl::Controls::TControl::WMLButtonDown
(00278809) Vcl::Controls::TWinControl::WndProc
(00277E50) Vcl::Controls::TWinControl::MainWndProc
(003A346C) System::Classes::_18201
(00045359) USER32.dll
(00036297) USER32.dll
(000353D6) USER32.dll
(000351AB) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.