Same for me here using disco...
update-initramfs: Generating /boot/initrd.img-4.18.0-10-generic Processing triggers for libc-bin (2.28-0ubuntu1) ... Errors were encountered while processing: nvidia-dkms-390 nvidia-driver-390 E: Sub-process /usr/bin/dpkg returned an error code (1)
Same for me here using disco...
update-initramfs: Generating /boot/initrd. img-4.18. 0-10-generic
Processing triggers for libc-bin (2.28-0ubuntu1) ...
Errors were encountered while processing:
nvidia-dkms-390
nvidia-driver-390
E: Sub-process /usr/bin/dpkg returned an error code (1)