Ubuntu 18.10 hangs after closing Unreal Engine 4.21

my Ubuntu 18.10 hangs after closing Unreal Engine 4.21. This started to happen when switched to vulkan. there were no such problems with opengl.
[link text][1]
[1]: 261952-ue4.log (190 KB)

Hello,

We’ve recently made a switch to a new bug reporting method using a more structured form. Please visit the link below for more details and report the issue using the new Bug Submission Form. Feel free to continue to use this thread for community discussion around the issue.

https://epicsupport.force.com/unrealengine/s/

Thanks