Index was outside the bounds of the array

public List<Item> GetTaggedRelatedItems()
        {
            List<Item> taggedRelatedItem = new List<Item>();
            try
            {
                const int totalItemCount = 4;
                var scContext = new SitecoreContext();
                List<Item> finalItems = new List<Item>();
                //Fetching the pubs and reports landing page from rendering para

                var topicField = Sitecore.Context.Item.Fields["Topic"];

                if (topicField != null)
                {
                    string[] housingPhaseTags = topicField.Value.Split(new char[] { '|' });
                    foreach (var tag in housingPhaseTags)
                    {

                        // Check for tags which are added as |Tag1|Tag2 instead of Tag1|Tag2
                        if (!string.IsNullOrWhiteSpace(tag))
                        {
                            var tagItem = Sitecore.Context.Database.GetItem(ID.Parse(tag));
                            if (tagItem != null && tagItem.Fields["RootNode"] != null)
                            {
                                var rootNode = Sitecore.Context.Database.GetItem(ID.Parse(tagItem.Fields["RootNode"].Value));
                                if (rootNode != null)
                                {
                                    //var relatedItems = rootNode.Axes.GetDescendants().Where(x => x.Visualization.GetLayout(Sitecore.Context.Device) != null);

                                    var childItems = contentSearchAPI.GetIndexForRootItem(rootNode, ID.Parse(Guid.Empty));
                                    if (childItems != null)
                                    {
                                        var relatedItems = childItems.Where(x => x != null && x.Visualization.GetLayout(Sitecore.Context.Device) != null); //## Content Search API ##

                                        var housingPhaseItems = relatedItems.Where(x => x.Fields["Topic"].Value.Contains(tag)).ToList();

                                        taggedRelatedItem.AddRange(housingPhaseItems);

                                        //Fetch the topic or subject tag in the given navigational bucket
                                        var topicTags = Context.Item.Fields["Tags"];
                                        if (topicTags != null && !string.IsNullOrWhiteSpace(topicTags.Value))
                                        {
                                            //Hack to make the tags work as the content migration script introduced extra | at the begining of the tags field.
                                            // So picking the [1] item as opposed [0] item.
                                            var firstTopicTag = topicTags.Value.Split(new char[] { '|' })[1];
                                            finalItems = housingPhaseItems.Where(x => x.Fields["Tags"].Value.Contains(firstTopicTag)).ToList();

                                            // Fetch remaining values from other topic or subject tags
                                            if (finalItems.Count == 0 || finalItems.Count < 4)
                                                finalItems.AddRange(GetSecondaryRelatedItem(housingPhaseItems, totalItemCount - finalItems.Count));

                                            finalItems.ForEach(x => taggedRelatedItem.Add(scContext.Cast<Item>(x)));
                                        }
                                    }
                                    else
                                    {
                                        Sitecore.Diagnostics.Log.Info("HCWebHelper -> GetTaggedRelatedItems - No Items(s) found from Index", this);
                                    }
                                }
                            }
                        }
                    }
                }

            }
            catch (Exception ex)
            {
                StackTrace stackTrace = new StackTrace(ex, true);
                var topFrame = stackTrace.GetFrame(1);
                var caller = topFrame != null ? topFrame.GetMethod()?.Name : string.Empty;
                var line = topFrame != null ? topFrame.GetFileLineNumber().ToString() : string.Empty;

                var exString = string.Format(@"Message : {0}, Caller: {1}, Line: {2}, StackTrace: {3}", ex.Message, caller, line, ex.StackTrace);

                Sitecore.Diagnostics.Log.Error(exString, this);
            }
            //Incase of empty taggedRelatedItem list it should return empty list<item>()  
            return (taggedRelatedItem != null && taggedRelatedItem.Count >= 4) ? taggedRelatedItem.Take(4).ToList() : new List<Item>();
        }