Adds some null checks, need to see if tests pass

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

@@ -1028,8 +1028,13 @@ namespace Umbraco.Web.PublishedCache.NuCache
         {
             parentLink = parentLink ?? GetRequiredParentLink(content, null);
 
+            // TODO: This can result in a null value? see https://github.com/umbraco/Umbraco-CMS/issues/7868
+            // It seems to be related to having corrupt Paths in the umbracoNode table.
             var parent = parentLink.Value;
 
+            if (parent == null)
+                throw new PanicException($"A null Value was returned on the {nameof(parentLink)} LinkedNode with id={content.ParentContentId}, potentially your database paths are corrupted, please see the HealthCheck dashboard and fixup data inconsistencies.");
+
             // if parent has no children, clone parent + add as first child
             if (parent.FirstChildContentId < 0)
             {

src/Umbraco.Web/PublishedCache/NuCache/Snap/LinkedNode.cs

@@ -1,4 +1,6 @@
-namespace Umbraco.Web.PublishedCache.NuCache.Snap
+using System;
+
+namespace Umbraco.Web.PublishedCache.NuCache.Snap
 {
     //NOTE: This cannot be struct because it references itself
 
@@ -11,7 +13,7 @@
     {
         public LinkedNode(TValue value, long gen, LinkedNode<TValue> next = null)
         {
-            Value = value;
+            Value = value ?? throw new ArgumentNullException(nameof(value));
             Gen = gen;
             Next = next;
         }