checkAPT: handle packages without a candidate version

Skip installed packages whose apt cache candidate is None instead of
crashing with 'NoneType has no attribute version'. This can happen when
a package is orphaned or removed from the configured repositories, and
previously caused the entire update list refresh to fail.

Closes #1059

Signed-off-by: SAY-5 <say.apm35@gmail.com>

Author: SAY-5

usr/lib/linuxmint/mintUpdate/aptUpdater.py

@@ -240,6 +240,12 @@ def find_changes(self):
 
         # Package updates
         for pkg in changes:
+            if pkg.candidate is None:
+                # Installed package has no candidate version (e.g. orphaned
+                # or removed from the repositories). Skip it so the rest of
+                # the update list can still be refreshed.
+                print("Skipping package without candidate version: %s" % pkg.name)
+                continue
             if (pkg.is_installed and pkg.candidate.version != pkg.installed.version):
                 if (pkg.marked_upgrade or pkg.marked_downgrade):
                     self.add_update(pkg)