I've been having this issue too with 4.0-5291 on Windows 8.1. The issue seems to happen every 5 or so isos. I've attatched the call stack when the error happens. It crashes in the specified function because m_rVolume is set to null when it tries to call its Read method.
DolphinD.exe!DiscIO::CFileSystemGCWii::Read32(unsigned __int64 _Offset) Line 203 C++
DolphinD.exe!DiscIO::CFileSystemGCWii::DetectFileSystem() Line 248 C++
DolphinD.exe!DiscIO::CFileSystemGCWii::CFileSystemGCWii(const DiscIO::IVolume * _rVolume) Line 28 C++
DolphinD.exe!DiscIO::CreateFileSystem(const DiscIO::IVolume * _rVolume) Line 22 C++
DolphinD.exe!DiscIO::DiscScrubber::ParsePartitionData(DiscIO::DiscScrubber::SPartition & _rPartition) Line 268 C++
DolphinD.exe!DiscIO::DiscScrubber::ParseDisc() Line 250 C++
DolphinD.exe!DiscIO::DiscScrubber::SetupScrub(const std::basic_string<char,std::char_traits,std::allocator > & filename, int block_size) Line 111 C++
DolphinD.exe!DiscIO::CompressFileToBlob(const std::basic_string<char,std::char_traits,std::allocator > & infile, const std::basic_string<char,std::char_traits,std::allocator > & outfile, unsigned int sub_type, int block_size, bool (const std::basic_string<char,std::char_traits,std::allocator > &, float, void *) * callback, void * arg) Line 159 C++
DolphinD.exe!CGameListCtrl::CompressSelection(bool _compress) Line 1206 C++
DolphinD.exe!CGameListCtrl::OnMultiCompressISO(wxCommandEvent & formal) Line 1144 C++
DolphinD.exe!wxAppConsoleBase::HandleEvent(wxEvtHandler * handler, void (wxEvent &) * func, wxEvent & event) Line 612 C++
DolphinD.exe!wxAppConsoleBase::CallEventHandler(wxEvtHandler * handler, wxEventFunctor & functor, wxEvent & event) Line 624 C++
DolphinD.exe!wxEvtHandler::ProcessEventIfMatchesId(const wxEventTableEntryBase & entry, wxEvtHandler * handler, wxEvent & event) Line 1386 C++
DolphinD.exe!wxEvtHandler::SearchDynamicEventTable(wxEvent & event) Line 1743 C++
DolphinD.exe!wxEvtHandler::TryHereOnly(wxEvent & event) Line 1577 C++
DolphinD.exe!wxEvtHandler::TryBeforeAndHere(wxEvent & event) Line 3671 C++
DolphinD.exe!wxEvtHandler::ProcessEventLocally(wxEvent & event) Line 1514 C++
DolphinD.exe!wxEvtHandler::ProcessEvent(wxEvent & event) Line 1487 C++
DolphinD.exe!wxEvtHandler::SafelyProcessEvent(wxEvent & event) Line 1605 C++
DolphinD.exe!wxWindowBase::HandleWindowEvent(wxEvent & event) Line 1526 C++
DolphinD.exe!wxMenuBase::SendEvent(int itemid, int checked) Line 674 C++
DolphinD.exe!wxMenu::MSWCommand(unsigned int formal, unsigned short id) Line 985 C++
DolphinD.exe!wxWindow::HandleCommand(unsigned short id, unsigned short cmd, HWND * control) Line 5302 C++
DolphinD.exe!wxWindow::MSWHandleMessage(__int64 * result, unsigned int message, unsigned __int64 wParam, __int64 lParam) Line 3048 C++
DolphinD.exe!wxWindow::MSWWindowProc(unsigned int message, unsigned __int64 wParam, __int64 lParam) Line 3628 C++
DolphinD.exe!wxListCtrl::MSWWindowProc(unsigned int nMsg, unsigned __int64 wParam, int64 lParam) Line 3061 C++
DolphinD.exe!wxWndProc(HWND * hWnd, unsigned int message, unsigned __int64 wParam, __int64 lParam) Line 2717 C++
[External Code]
DolphinD.exe!wxYieldForCommandsOnly() Line 2219 C++
DolphinD.exe!wxWindow::DoPopupMenu(wxMenu * menu, int x, int y) Line 2264 C++
DolphinD.exe!wxWindowBase::PopupMenu(wxMenu * menu, int x, int y) Line 2961 C++
DolphinD.exe!wxWindowBase::PopupMenu(wxMenu * menu, const wxPoint & pos) Line 1209 C++
DolphinD.exe!CGameListCtrl::OnRightClick(wxMouseEvent & event) Line 987 C++
DolphinD.exe!wxAppConsoleBase::HandleEvent(wxEvtHandler * handler, void (wxEvent &) * func, wxEvent & event) Line 612 C++
DolphinD.exe!wxAppConsoleBase::CallEventHandler(wxEvtHandler * handler, wxEventFunctor & functor, wxEvent & event) Line 624 C++
DolphinD.exe!wxEvtHandler::ProcessEventIfMatchesId(const wxEventTableEntryBase & entry, wxEvtHandler * handler, wxEvent & event) Line 1386 C++
DolphinD.exe!wxEvtHandler::SearchDynamicEventTable(wxEvent & event) Line 1743 C++
DolphinD.exe!wxEvtHandler::TryHereOnly(wxEvent & event) Line 1577 C++
DolphinD.exe!wxEvtHandler::TryBeforeAndHere(wxEvent & event) Line 3671 C++
DolphinD.exe!wxEvtHandler::ProcessEventLocally(wxEvent & event) Line 1514 C++
DolphinD.exe!wxEvtHandler::ProcessEvent(wxEvent & event) Line 1487 C++
DolphinD.exe!wxEvtHandler::SafelyProcessEvent(wxEvent & event) Line 1605 C++
DolphinD.exe!wxWindowBase::HandleWindowEvent(wxEvent & event) Line 1526 C++
DolphinD.exe!wxWindow::HandleMouseEvent(unsigned int msg, int x, int y, unsigned int flags) Line 5508 C++
DolphinD.exe!wxWindow::MSWHandleMessage(__int64 * result, unsigned int message, unsigned __int64 wParam, __int64 lParam) Line 3008 C++
DolphinD.exe!wxWindow::MSWWindowProc(unsigned int message, unsigned __int64 wParam, __int64 lParam) Line 3628 C++
DolphinD.exe!wxListCtrl::MSWWindowProc(unsigned int nMsg, unsigned __int64 wParam, int64 lParam) Line 3061 C++
DolphinD.exe!wxWndProc(HWND * hWnd, unsigned int message, unsigned int64 wParam, int64 lParam) Line 2717 C++
[External Code]
DolphinD.exe!wxWindow::MSWProcessMessage(tagMSG * pMsg) Line 2501 C++
DolphinD.exe!wxGUIEventLoop::PreProcessMessage(tagMSG * msg) Line 148 C++
DolphinD.exe!wxGUIEventLoop::ProcessMessage(tagMSG * msg) Line 166 C++
DolphinD.exe!wxGUIEventLoop::Dispatch() Line 232 C++
DolphinD.exe!wxEventLoopManual::ProcessEvents() Line 173 C++
DolphinD.exe!wxEventLoopManual::DoRun() Line 206 C++
DolphinD.exe!wxEventLoopBase::Run() Line 78 C++
DolphinD.exe!wxAppConsoleBase::MainLoop() Line 334 C++
DolphinD.exe!wxAppConsoleBase::OnRun() Line 260 C++
DolphinD.exe!wxAppBase::OnRun() Line 305 C++
DolphinD.exe!wxEntryReal(int & argc, wchar_t * * argv) Line 495 C++
DolphinD.exe!wxEntry(int & argc, wchar_t * * argv) Line 188 C++
DolphinD.exe!wxEntry(HINSTANCE * hInstance, HINSTANCE * formal, char * formal, int nCmdShow) Line 415 C++
DolphinD.exe!WinMain(HINSTANCE * hInstance, HINSTANCE * hPrevInstance, char * __formal, int nCmdShow) Line 90 C++
[External Code]