[MM-52955] Fix panic for not found posts (#23561)

server/channels/store/sqlstore/reaction_store.go

@@ -41,6 +41,11 @@ func (s *SqlReactionStore) Save(reaction *model.Reaction) (re *model.Reaction, e
 		if err != nil {
 			return nil, errors.Wrap(err, "failed while getting channelId from Posts")
 		}
+
+		if len(channelIds) == 0 {
+			return nil, store.NewErrNotFound("Post", reaction.PostId)
+		}
+
 		reaction.ChannelId = channelIds[0]
 	}
 	err = s.saveReactionAndUpdatePost(transaction, reaction)

server/channels/store/storetest/reaction_store.go

@@ -118,6 +118,16 @@ func testReactionSave(t *testing.T, ss store.Store) {
 	_, nErr = ss.Reaction().Save(reaction5)
 	require.Error(t, nErr, "should've failed for invalid reaction")
 
+	t.Run("channel not found", func(t *testing.T) {
+		// invalid reaction
+		reaction5 := &model.Reaction{
+			UserId:    reaction1.UserId,
+			PostId:    model.NewId(), // Unknown PostId
+			EmojiName: model.NewId(),
+		}
+		_, nErr = ss.Reaction().Save(reaction5)
+		require.Error(t, nErr, "should've failed because postId doesn't belong to a stored post")
+	})
 }
 
 func testReactionDelete(t *testing.T, ss store.Store) {