Crash #2, as seen by DrMemory:

Error #4: INVALID HEAP ARGUMENT to free 0x02a40fd0
# 0 replace_operator_delete_nothrow                            [d:\drmemory_package\common\alloc_replace.c:2974]
# 1 operator delete                                            [f:\dd\vctools\crt\vcstartup\src\heap\delete_scalar_size.cpp:15]
# 2 QActionPrivate::~QActionPrivate                            [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qaction.cpp:103]
# 3 QActionPrivate::`scalar deleting destructor'
# 4 QAction::~QAction                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qaction.cpp:676]
# 5 QAction::`scalar deleting destructor'
# 6 QWidget::~QWidget                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:1679]
# 7 QStackedWidget::~QStackedWidget                            [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qstackedwidget.cpp:196]
# 8 QWidget::~QWidget                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:1679]
# 9 QStackedWidget::~QStackedWidget                            [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qstackedwidget.cpp:196]
#10 WWidgetGroup::~WWidgetGroup
#11 WWidgetGroup::`scalar deleting destructor'
#12 QObjectPrivate::deleteChildren                             [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\corelib\kernel\qobject.cpp:1935]
#13 QWidget::~QWidget                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:1679]
#14 QSplitter::~QSplitter                                      [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qsplitter.cpp:1056]
#15 WSplitter::`scalar deleting destructor'
#16 QWidget::~QWidget                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:1679]
#17 QStackedWidget::~QStackedWidget                            [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qstackedwidget.cpp:196]
#18 WWidgetGroup::~WWidgetGroup
#19 WWidgetGroup::`scalar deleting destructor'
#20 QObjectPrivate::deleteChildren                             [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\corelib\kernel\qobject.cpp:1935]
#21 QWidget::~QWidget                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:1679]
#22 QStackedWidget::~QStackedWidget                            [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qstackedwidget.cpp:196]
#23 WWidgetGroup::~WWidgetGroup
#24 WWidgetGroup::`scalar deleting destructor'
#25 QObjectPrivate::deleteChildren                             [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\corelib\kernel\qobject.cpp:1935]
#26 QWidget::~QWidget                                          [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:1679]
#27 QStackedWidget::~QStackedWidget                            [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qstackedwidget.cpp:196]
#28 WWidgetGroup::~WWidgetGroup
#29 WWidgetGroup::`scalar deleting destructor'
#30 QObject::event                                             [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\corelib\kernel\qobject.cpp:1203]
#31 QWidget::event                                             [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qwidget.cpp:8859]
#32 QFrame::event                                              [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\widgets\qframe.cpp:557]
#33 WLibrary::event                                            [c:\mixxx\environments\mixxx-static-x86\src\widget\wlibrary.cpp:74]
#34 QApplicationPrivate::notify_helper                         [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qapplication.cpp:4565]
#35 QApplication::notify                                       [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\gui\kernel\qapplication.cpp:4426]
#36 MixxxApplication::notify                                   [c:\mixxx\environments\mixxx-static-x86\src\mixxxapplication.cpp:137]
#37 QCoreApplication::notifyInternal                           [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\corelib\kernel\qcoreapplication.cpp:955]
#38 QCoreApplicationPrivate::sendPostedEvents                  [c:\mixxx\environments\msvc15-static-x86-release\build\qt-everywhere-opensource-src-4.8.7\src\corelib\kernel\qcoreapplication.cpp:1579]
#39 MixxxMainWindow::finalize                                  [c:\mixxx\environments\mixxx-static-x86\src\mixxx.cpp:476]
Note: @0:03:55.031 in thread 5764