fix: remove warning

Author: Tr0sT

Packages/com.nuclearband.sodatabase/Editor/ScriptableObjectDatabaseEditorWindow.cs

@@ -259,7 +259,7 @@ private void Save()
             AssetDatabase.Refresh();
         }
 
-        static IEnumerable<OdinMenuItem> Flatten(List<OdinMenuItem> collection)
+        private static IEnumerable<OdinMenuItem> Flatten(IEnumerable<OdinMenuItem> collection)
         {
             foreach (var o in collection)
             {

Packages/com.nuclearband.sodatabase/Runtime/SerializationUtilities/DataNodeReferenceResolver.cs

@@ -6,7 +6,7 @@
 public class DataNodeReferenceResolver : IReferenceResolver
 {
     public static DataNode CurrentDataNode = null!;
-    
+
     public object ResolveReference(object context, string reference)
     {
         try
@@ -15,7 +15,7 @@ public object ResolveReference(object context, string reference)
         }
         catch (Exception)
         {
-            return null;
+            return null!;
         }
     }