Things I did to successfully run Kali Linux in VirtualBox 5.2 on Ubuntu 18.04, not in this order:

1. Add Oracle repositories
2. Install virtualbox-5.2, virtualbox-dkms, virtualbox-ext-pack.
3. Reboot and follow the prompts to add MOK (thanks to virtualbox-dkms)
4. Reboot again, enter BIOS, and enable VT under Virtualization
5. Download and install Kali Linux VirtualBox Image 64bit
6. Change the machine settings:
   • System > Memory > 2048 MB
   • System > Processor > set 1 CPU
   • System > Display > Disable 3D Acceleration