Commenting out the ddebs lines, I get:

apport-retrace -v -s -R -S system ./_usr_bin_gnome-shell-modified.1000.crash 
Get:1 http://ppa.launchpad.net/saiarcot895/chromium-dev/ubuntu focal InRelease [20.8 kB]                                                                                                                  
Get:2 https://mirror.init7.net/ubuntu focal InRelease [255 kB]                                                                                                                                            
Get:3 http://ppa.launchpad.net/saiarcot895/chromium-dev/ubuntu focal/main amd64 Packages [1604 B]                                                                                                         
Get:4 https://mirror.init7.net/ubuntu focal-updates InRelease [79.7 kB]                                                                                                                                   
Get:5 http://ppa.launchpad.net/saiarcot895/chromium-dev/ubuntu focal/main Translation-en [1084 B]                                                                                                         
Get:6 https://mirror.init7.net/ubuntu focal-backports InRelease [79.7 kB]                                                                                                                                 
Get:7 https://mirror.init7.net/ubuntu focal-security InRelease [79.7 kB]                                                                                                                                  
Get:8 https://mirror.init7.net/ubuntu focal/restricted Sources [7032 B]                                                                                                                                   
Get:9 https://mirror.init7.net/ubuntu focal/multiverse Sources [176 kB]                                                                                                                                   
Get:10 https://mirror.init7.net/ubuntu focal/main Sources [838 kB]                                                                                                                                        
Get:11 https://mirror.init7.net/ubuntu focal/universe Sources [9792 kB]                                                                                                                                   
Get:12 https://mirror.init7.net/ubuntu focal/main amd64 Packages [972 kB]                                                                                                                                 
Get:13 https://mirror.init7.net/ubuntu focal/main Translation-en [500 kB]                                                                                                                                 
Get:14 https://mirror.init7.net/ubuntu focal amd64 Contents (deb) [41.3 MB]                                                                                                                               
Get:15 https://mirror.init7.net/ubuntu focal/restricted amd64 Packages [21.4 kB]                                                                                                                          
Get:16 https://mirror.init7.net/ubuntu focal/restricted Translation-en [6136 B]                                                                                                                           
Get:17 https://mirror.init7.net/ubuntu focal/universe amd64 Packages [8651 kB]                                                                                                                            
Get:18 https://mirror.init7.net/ubuntu focal/universe Translation-en [5161 kB]                                                                                                                            
Get:19 https://mirror.init7.net/ubuntu focal/multiverse amd64 Packages [145 kB]                                                                                                                           
Get:20 https://mirror.init7.net/ubuntu focal/multiverse Translation-en [106 kB]                                                                                                                           
Fetched 68.2 MB in 6s (1020 kB/s)                                                                                                                                                                         
Fetched 0 B in 0s (0 B/s)                                                                                                                                                                                 
Extracting downloaded debs...
Traceback (most recent call last):
  File "/usr/bin/apport-retrace", line 322, in <module>
    sandbox, cache, outdated_msg = apport.sandboxutils.make_sandbox(
  File "/usr/lib/python3/dist-packages/apport/sandboxutils.py", line 220, in make_sandbox
    pkgs = needed_runtime_packages(report, sandbox_dir, pkgmap_cache_dir, pkg_versions, verbose)
  File "/usr/lib/python3/dist-packages/apport/sandboxutils.py", line 92, in needed_runtime_packages
    pkg = apport.packaging.get_file_package(l, True, pkgmap_cache_dir,
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 490, in get_file_package
    return self._search_contents(file, map_cachedir, release, arch)
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 1296, in _search_contents
    url = '%s/dists/%s%s/Contents-%s.gz' % (self._get_mirror(), release, pocket, arch)
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 1252, in _get_mirror
    self._mirror = self._get_primary_mirror_from_apt_sources('/etc/apt/sources.list')
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 1242, in _get_primary_mirror_from_apt_sources
    raise SystemError('cannot determine default mirror: %s does not contain a valid deb line'
SystemError: cannot determine default mirror: /etc/apt/sources.list does not contain a valid deb line