Invalid access to memory - TSynchronizeChecklist::GetCount

<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 6.0

Error message:
Invalid access to memory.

Stack trace:
(00D303FE) TSynchronizeChecklist::GetCount
(000974CF) ntdll.dll
(00077751) ntdll.dll.KiUserExceptionDispatcher
(00D306EE) TSynchronizeProgress::GetProcessed
(00D3073E) TSynchronizeProgress::Progress
(00E96776) TProgressForm::UpdateControls
(00E972D8) TProgressForm::UpdateTimerTimer
(00213A0C) System::Classes::_18201
(000313E1) USER32.dll
(000220E1) USER32.dll
(000209A0) USER32.dll
(000204EB) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.

<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 6.1.1

Error message:
メモリのアクセス違反

スタック トレース:
(00D3001A) TSynchronizeChecklist::GetCount
(000825EF) ntdll.dll
(0007015A) ntdll.dll.KiUserExceptionDispatcher
(00D3030A) TSynchronizeProgress::GetProcessed
(00D3035A) TSynchronizeProgress::Progress
(00E967DA) TProgressForm::UpdateControls
(00E97360) TProgressForm::UpdateTimerTimer
(00214D98) System::Classes::_18201
(0002C2D1) USER32.dll
(0000C305) USER32.dll
(0000BC4F) USER32.dll
(0000BA0B) USER32.dll.DispatchMessageW
(004C56E9) Tb2extitems::_16456
(004C5937) Tb2extitems::TTBEditItemViewer::EditLoop
(004C5B2E) Tb2extitems::TTBEditItemViewer::DoExecute
(004D535E) Tb2item::TTBModalHandler::Loop
(004D44EF) Tb2item::TTBView::EnterToolbarLoop
(004E203A) Tb2toolbar::TTBToolbarView::EnterToolbarLoop
(003F1DDC) Vcl::Controls::TControl::DoMouseDown
(003F1E2B) Vcl::Controls::TControl::WMLButtonDown
(003F62DD) Vcl::Controls::TWinControl::WndProc
(003F5924) Vcl::Controls::TWinControl::MainWndProc
(00214D98) System::Classes::_18201
(0002C2D1) USER32.dll
(0000C305) USER32.dll
(0000BC4F) USER32.dll
(0000BA0B) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.