Segmentation Fault trying to display camera feed

[Xenoshell]

Hello,
I am working with PyQt5 version 5.7 , Qt5 version 5.71, Python 3.5 and Raspbian/Raspberry Pi. I already debugged the segfault with gdb. The result is :

(gdb) bt
0  0x733d4620 in platform_get_handle () from /opt/vc/lib/libEGL.so
1  0x733c9f2c in eglCreateWindowSurface () from /opt/vc/lib/libEGL.so
2  0x722e7004 in ?? ()
   from /usr/lib/arm-linux-gnueabihf/qt5/plugins/xcbglintegrations/libqxcb-egl-integration.so
Backtrace stopped: previous frame identical to this frame (corrupt stack?)

Sadly i am not really good in interpreting debug output so i cant really do something with that.
My whole code can be found here. I know the problem is at:

self.camvfind.setVisible(True)
self.cam.setViewfinder(self.camvfind)

because of just commenting some lines and looking if the error would still occur.
My main problem is that somehow the Segmentation Fault doesnt always happen. Sometimes i really get camera feed but only sometimes.
Any help is appreciated and if you have questions just ask-

Greets,

Xenoshell