<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 5.11.3

Error message:
Stack trace:
(0017F057) Vcl::Themes::TArray::BinarySearch<>
(0017F057) Vcl::Themes::TArray::BinarySearch<>
(006DF323) TTerminalQueue::Idle
(00096CC9) TTerminalManager::Idle
(00017207) TCustomScpExplorerForm::Idle
(0003DCDF) TNonVisualDataModule::DoIdle
(0003DC9C) TNonVisualDataModule::SessionIdleTimerTimer
(0037BCBC) System::Classes::_18201
(00007731) USER32.dll
(00007811) USER32.dll
(000079C8) USER32.dll
(00007A0B) USER32.dll.DispatchMessageW

(0089D9B5) C5067_0
(0089EF8F) _ReThrowException
(00096CE5) TTerminalManager::Idle
(00017207) TCustomScpExplorerForm::Idle
(0003DCDF) TNonVisualDataModule::DoIdle
(0003DC9C) TNonVisualDataModule::SessionIdleTimerTimer
(0037BCBC) System::Classes::_18201
(00007731) USER32.dll
(00007811) USER32.dll
(000079C8) USER32.dll
(00007A0B) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.