fix: unlink broken link (#37329)

This fixes a bug that occurs when unlinking a block from a downstream block that has a broken link

cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py

@@ -456,25 +456,23 @@ class DownstreamView(DeveloperErrorViewMixin, APIView):
         """
         downstream = _load_accessible_block(request.user, usage_key_string, require_write_access=True)
         affected_blocks: list[XBlock] = []
+        # Get the upstream ref before severing the link, so we can delete
+        # the corresponding ComponentLink or ContainerLink below.
+        upstream_ref = downstream.upstream
         try:
-            # Try to get the upstream key before severing the link, so we can delete
-            # the corresponding ComponentLink or ContainerLink below.
-            try:
-                upstream_key = UpstreamLink.get_for_block(downstream).upstream_key
-            except NoUpstream:
-                # Even if we don't have an UpstreamLink, we still need to check
-                # if the block has the upstream key set, so we don't want to
-                # raise an exception here.
-                upstream_key = None
 
             affected_blocks = sever_upstream_link(downstream)
 
             # Remove the ComponentLink or ContainerLink, if it exists.
-            if upstream_key:
-                if isinstance(upstream_key, LibraryUsageLocatorV2):
+            if upstream_ref:
+                try:
                     ComponentLink.get_by_downstream_usage_key(downstream.usage_key).delete()
-                elif isinstance(upstream_key, LibraryContainerLocator):
-                    ContainerLink.get_by_downstream_usage_key(downstream.usage_key).delete()
+                except ComponentLink.DoesNotExist:
+                    try:
+                        ContainerLink.get_by_downstream_usage_key(downstream.usage_key).delete()
+                    except ContainerLink.DoesNotExist:
+                        # If neither link exists, that's fine--we just wanted to clean up if possible.
+                        pass
         except NoUpstream:
             logger.exception(
                 "Tried to DELETE upstream link of '%s', but it wasn't linked to anything in the first place. "