Minor editor tweaks

Source/Editor/Content/Items/JsonAssetItem.cs

@@ -11,7 +11,10 @@ namespace FlaxEditor.Content
     /// <seealso cref="FlaxEditor.Content.AssetItem" />
     public class JsonAssetItem : AssetItem
     {
-        private readonly SpriteHandle _thumbnail;
+        /// <summary>
+        /// Asset icon.
+        /// </summary>
+        protected SpriteHandle _thumbnail;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonAssetItem"/> class.

Source/Editor/Content/Proxy/JsonAssetProxy.cs

@@ -127,7 +127,7 @@ namespace FlaxEditor.Content
     /// Generic Json assets proxy (supports all json assets that don't have dedicated proxy).
     /// </summary>
     /// <seealso cref="FlaxEditor.Content.JsonAssetBaseProxy" />
-    public sealed class GenericJsonAssetProxy : JsonAssetProxy
+    public class GenericJsonAssetProxy : JsonAssetProxy
     {
         /// <inheritdoc />
         public override string TypeName => typeof(JsonAsset).FullName;
@@ -161,7 +161,7 @@ namespace FlaxEditor.Content
     /// Content proxy for a json assets of the given type that can be spawned in the editor.
     /// </summary>
     /// <seealso cref="FlaxEditor.Content.JsonAssetProxy" />
-    public sealed class SpawnableJsonAssetProxy<T> : JsonAssetProxy where T : new()
+    public class SpawnableJsonAssetProxy<T> : JsonAssetProxy where T : new()
     {
         /// <inheritdoc />
         public override string Name { get; } = Utilities.Utils.GetPropertyNameUI(typeof(T).Name);

Source/Editor/Windows/ContentWindow.ContextMenu.cs

@@ -155,7 +155,7 @@ namespace FlaxEditor.Windows
             var scriptType = new ScriptType(typeof(Script));
             foreach (var type in Editor.CodeEditing.All.Get())
             {
-                if (type.IsAbstract)
+                if (type.IsAbstract || type.Type == null)
                     continue;
                 if (actorType.IsAssignableFrom(type) || scriptType.IsAssignableFrom(type))
                     continue;