Installer stopped because it couldn't install grub.

Installing xubuntu from a flash drive when the error occurred.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
Date: Tue Mar 19 10:59:16 2019
InstallCmdLine: file=/cdrom/preseed/xubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity
LiveMediaBuild: Xubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)