manifest: Skip updating appdata if last source is not external

Closes #346

src/manifest.py

@@ -486,10 +486,18 @@ def _update_appdata(self):
             elif data.source_path == self._root_manifest_path:
                 selected_data = data
         else:
-            # Guess that the last external source in the root manifest is the one
-            # corresponding to the main application bundle.
-            assert selected_data is not None
-            log.warning("Guessed upstream source: %s", selected_data)
+            log.warning(
+                "No main source found. Assuming the last source is the main source"
+            )
+            if selected_data is not None:
+                # Guess that the last external source in the root manifest is the one
+                # corresponding to the main application bundle.
+                log.warning("Guessed upstream source: %s", selected_data)
+            else:
+                log.error(
+                    "The last source is not external. Appdata will not be updated"
+                )
+                return
 
         last_update = selected_data.new_version