I am using Virtual Box Graphical User Interface Version 5.2.18_Ubuntu r123745 on Ubuntu 18.04.2 LTS. When loading into my install the screen flashes black and it sometimes only flashes or shows part of the screen.
I have 8GB RAM, 4 processes, 128MB Vid Mem, 3D Acceleration enabled, and 20GB assigned to this environment.
This doesn't happen in my other Linux VMs with 2GB RAM, 2 Processor, 128 MB Vid Mem, 3D Acceleration enabled. ( Kali )
I seen this https://tracker.pureos.net/T538 and that is when I assigned more RAM and CPU to it, but it didn't fix it.