Just try re-running it. I have seen this happen on occasion and it seems to be a small timing hole.
Per Bryan's suggestion, re-running the process seemed to work. And yes, it took a Really Long Time. However, my 2015 iMac, where I'm running VirtualBox and InstallerApp2ISO, is booting Mojave from an external Thunderbolt 2 La Cie drive, so that's probably part of the slowness..
I'm on the second re-boot where the Mojave installer is supposed to "complete" its process.
It's moving VERY slowly,
progressing around 15% after 2 full hours! Although the installer says "14 minutes remaining", at this rate, it may take around another 8-10 hours.
UPDATE: After 12 hours, the Mojave didn't get past 40%, so I stopped the VM, and set the CPUs to 3 and doubled the memory to 8GB. That did the trick, and I was able to install Mojave 10.14.6. I have to say it runs nicely.
I was able to mount the Guest Additions from 6.0.14, however when attempting to install them, I got a "System Extension Blocked" alert: "A program tried to load one or more system extensions that are incompatible with this version of MacOS. Please contact Oracle America, Inc" for support". I'll open a new post for that issue.
Stephen