installation crashed when trying to install grub bootloader

[Zoltan]

installation crashed when trying to install grub bootloader onto windows partition
as a result computer is not able to boot the previously existing windows system, nor the kubuntu system installed during installation, booting stops at grub command line

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.12
ProcVersionSignature: Ubuntu 4.18.0-15.16~18.04.1-generic 4.18.20
Uname: Linux 4.18.0-15-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: KDE
Date: Wed Mar 27 23:36:31 2019
InstallCmdLine: file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity initrd=/casper/initrd quiet splash --- maybe-ubiquity
LiveMediaBuild: Kubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)