Fixes issue with nucache and rebuilding and fixes ChildrenForAllCultures to deal with unpublished children

src/Umbraco.Web/PublishedCache/NuCache/ContentStore.cs

@@ -647,9 +647,16 @@ namespace Umbraco.Web.PublishedCache.NuCache
             var id = content.FirstChildContentId;
             while (id > 0)
             {
-                var link = GetLinkedNode(id, "child");
-                ClearBranchLocked(link.Value);
-                id = link.Value.NextSiblingContentId;
+                if (TryGetLinkedNode(id, out var link))
+                {
+                    ClearBranchLocked(link.Value);
+                    id = link.Value.NextSiblingContentId;
+                }
+                else
+                {
+                    // break i guess?
+                    id = 0;
+                }
             }
         }
 
@@ -663,6 +670,18 @@ namespace Umbraco.Web.PublishedCache.NuCache
             throw new Exception($"panic: failed to get {description} with id={id}");
         }
 
+        private bool TryGetLinkedNode(int id, out LinkedNode<ContentNode> node)
+        {
+            if (_contentNodes.TryGetValue(id, out var link) && link.Value != null)
+            {
+                node = link;
+                return true;
+            }
+
+            node = null;
+            return false;
+        }
+
         private LinkedNode<ContentNode> GetParentLink(ContentNode content)
         {
             _contentNodes.TryGetValue(content.ParentContentId, out var link); // else null

src/Umbraco.Web/PublishedCache/NuCache/PublishedContent.cs

@@ -277,10 +277,43 @@ namespace Umbraco.Web.PublishedCache.NuCache
                 while (id > 0)
                 {
                     var content = getById(publishedSnapshot, IsPreviewing, id);
-                    if (content == null)
-                        throw new Exception($"panic: failed to get content with id={id}");
 
-                    yield return content;
+                    if (content != null)
+                    {
+                        yield return content;
+                    }
+                    else
+                    {
+                        //Why were we throwing here? It is perfectly legitimate that a child is not published and when IsPreviewing == false
+                        //this will return null even if the item is in nucache but only with an unpublished flag.
+                        //If we want to be very clear about something being wrong, then perhaps the exception should be thrown from within
+                        //the Func that gets the child where it can validate if there really is nothing there when something is expected?
+
+                        //In the meantime, we cannot continue so we should break?... BUT doesn't that mean that if there was a sibling next to this that was published
+                        //that it will now be excluded?
+
+                        //Well, in that case this is annoying, so the only thing i can think of is to get the preview version of it to get the
+                        //next child and continue that way?
+
+                        if (IsPreviewing)
+                        {
+                            //if we're in preview mode and nothing is returned then something is wrong
+                            throw new Exception($"panic: failed to get content with id={id}");
+                        }
+                        else
+                        {
+                            //get the preview version since this item might not be published
+                            content = getById(publishedSnapshot, true, id);
+                            if (content == null)
+                            {
+                                //if we're in preview mode and nothing is returned then something is wrong
+                                throw new Exception($"panic: failed to get content with id={id}");
+                            }
+
+                            //now we can continue with the next sibling id, but we aren't going to return this content item because it's not published
+                            //and we're not previewing.
+                        }   
+                    }
 
                     id = UnwrapIPublishedContent(content)._contentNode.NextSiblingContentId;
                 }