Encountered "/usr/sbin/grub-mkconfig: 8: /etc/default/grub: 4: not found" error while updating

Whenever I use

sudo apt-get upgrade

I am getting these errors:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages will be REMOVED:
  linux-image-4.15.0-46-generic
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
3 not fully installed or removed.
After this operation, 8,311 kB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 256191 files and directories currently installed.)
Removing linux-image-4.15.0-46-generic (4.15.0-46.49) ...
/etc/kernel/postrm.d/initramfs-tools:
update-initramfs: Deleting /boot/initrd.img-4.15.0-46-generic
/etc/kernel/postrm.d/zz-update-grub:
Sourcing file `/etc/default/grub'
/usr/sbin/grub-mkconfig: 8: /etc/default/grub: 4: not found
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 127
dpkg: error processing package linux-image-4.15.0-46-generic (--remove):
 installed linux-image-4.15.0-46-generic package post-removal script subprocess returned error exit status 1
Errors were encountered while processing:
 linux-image-4.15.0-46-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)

Output of cat /etc/default/grub:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT= 4
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

These errors are due to a space in line 8 in /etc/default/grub file. GRUB would think that it's some kind of argument instead of value. You need to remove that.

• Open /etc/default/grub with sudo privileges using nano (or pico, whichever you prefer)

  sudo nano /etc/default/grub
  

• Delete the space in line 8 before 4
• Save file using Ctrl+X followed by Y

• Update GRUB

  sudo update-grub
  

• Retry autoremove command