CV2 needs to be run on main thread.

2018-12-18 22:34:11.758 python[7689:373478] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'NSWindow drag regions should only be invalidated on the Main Thread!' *** First throw call stack: ( 0 CoreFoundation 0x00007fff3979843d __exceptionPreprocess + 256 1 libobjc.A.dylib 0x00007fff656a5720 objc_exception_throw + 48 2 CoreFoundation 0x00007fff397b1ec1 -[NSException raise] + 9 3 AppKit 0x00007fff36cb62a5 -[NSWindow(NSWindow_Theme) _postWindowNeedsToResetDragMarginsUnlessPostingDisabled] + 324 4 AppKit 0x00007fff36cb368c -[NSWindow _initContent:styleMask:backing:defer:contentView:] + 1488 5 AppKit 0x00007fff36d73edf -[NSPanel _initContent:styleMask:backing:defer:contentView:] + 50 6 AppKit 0x00007fff36cb30b6 -[NSWindow initWithContentRect:styleMask:backing:defer:] + 45 7 AppKit 0x00007fff36d73e94 -[NSPanel initWithContentRect:styleMask:backing:defer:] + 64 8 QtGui 0x000000010c224028 -[QCocoaPanel initWithContentRect:styleMask:backing:defer:] + 74 9 QtGui 0x000000010c22abbf -[NSWindow(QWidgetIntegration) qt_initWithQWidget:contentRect:styleMask:] + 81 10 QtGui 0x000000010c21be1c _ZL20qt_mac_create_windowP7QWidgetjmRK5QRect + 427 11 QtGui 0x000000010c21baa1 _ZN14QWidgetPrivate18qt_create_root_winEv + 65 12 QtGui 0x000000010c21d8d3 _ZN14QWidgetPrivate10create_sysElbb + 1079 13 QtGui 0x000000010c2b3d25 _ZN7QWidget6createElbb + 587 14 QtGui 0x000000010c2b29f6 _ZN14QWidgetPrivate4initEP7QWidget6QFlagsIN2Qt10WindowTypeEE + 362 15 QtGui 0x000000010c2b27e0 _ZN7QWidgetC2EPS_6QFlagsIN2Qt10WindowTypeEE + 122 16 QtGui 0x000000010c235cca _ZN14QDesktopWidgetC2Ev + 32 17 QtGui 0x000000010c279183 _ZN12QApplication7desktopEv + 53 18 QtGui 0x000000010c231a05 _Z9flipPointRK7CGPoint + 32 19 QtGui 0x000000010c211961 _ZN7QCursor3posEv + 47 20 QtGui 0x000000010c2890c5 _ZN11QMouseEventC2EN6QEvent4TypeERK6QPointN2Qt11MouseButtonE6QFlagsIS6_ES7_INS5_16KeyboardModifierEE + 79 21 QtGui 0x000000010c79e426 _ZN20QGraphicsViewPrivateC2Ev + 340 22 QtGui 0x000000010c7a19d5 _ZN13QGraphicsViewC2EP7QWidget + 37 23 cv2.so 0x000000010e74d27f _ZN15DefaultViewPortC2EP8CvWindowi + 31 24 cv2.so 0x000000010e7488ed _ZN8CvWindowC2E7QStringi + 397 25 cv2.so 0x000000010e740913 _ZN11GuiReceiver12createWindowE7QStringi + 227 26 cv2.so 0x000000010e74076c cvNamedWindow + 540 27 cv2.so 0x000000010e742ea1 _ZN11GuiReceiver9showImageE7QStringPv + 161 28 cv2.so 0x000000010e742d4c cvShowImage + 572 29 cv2.so 0x000000010e73b46b _ZN2cv6imshowERKNS_6StringERKNS_11_InputArrayE + 475 30 cv2.so 0x000000010dc73964 _ZL18pyopencv_cv_imshowP7_objectS0_S0_ + 404 31 Python 0x00007fff43b12bf0 PyEval_EvalFrameEx + 1102 32 Python 0x00007fff43b12588 PyEval_EvalCodeEx + 1540 33 Python 0x00007fff43ab9d88 PyFunction_SetClosure + 797 34 Python 0x00007fff43a9c9a7 PyObject_Call + 97 35 Python 0x00007fff43b159a9 PyEval_EvalFrameEx + 12807 36 Python 0x00007fff43b1873e _PyEval_SliceIndex + 404 37 Python 0x00007fff43b1645a PyEval_EvalFrameEx + 15544 38 Python 0x00007fff43b1873e _PyEval_SliceIndex + 404 39 Python 0x00007fff43b1645a PyEval_EvalFrameEx + 15544 40 Python 0x00007fff43b12588 PyEval_EvalCodeEx + 1540 41 Python 0x00007fff43ab9d88 PyFunction_SetClosure + 797 42 Python 0x00007fff43a9c9a7 PyObject_Call + 97 43 Python 0x00007fff43aa7053 PyMethod_New + 1176 44 Python 0x00007fff43a9c9a7 PyObject_Call + 97 45 Python 0x00007fff43b18158 PyEval_CallObjectWithKeywords + 159 46 Python 0x00007fff43b4482c initthread + 2817 47 libsystem_pthread.dylib 0x00007fff6696533d _pthread_body + 126 48 libsystem_pthread.dylib 0x00007fff669682a7 _pthread_start + 70 49 libsystem_pthread.dylib 0x00007fff66964425 thread_start + 13 ) libc++abi.dylib: terminating with uncaught exception of type NSException