Failed to install grub2

I am not sure, but it looks like it did not find the Grub installation target.

Note:
I had deleted some UEFI boot entries of an old Kubuntu install before installing Xubuntu 18.04.

Right after, i tried to install the Ubuntu 18.04 LTS, and it has been succeeded, but in this case i choose the exact EFI partition (/dev/sda3) for it to avoid the same problem here reported. In previous versions, that choice was not necessary, which is why i think it is a bug.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
Date: Fri Sep 21 15:53:24 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/xubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)