Minor refactor changes

Author: arimger

Assets/Editor Toolbox/Editor/ToolboxDrawerModule.cs

@@ -43,7 +43,6 @@ internal static void InitializeModule()
         /// </summary>
         private static readonly Dictionary<string, ToolboxPropertyHandler> propertyHandlers = new Dictionary<string, ToolboxPropertyHandler>();
 
-
         /// <summary>
         /// Settings provided to handle custom drawers.
         /// </summary>
@@ -54,15 +53,14 @@ internal static void InitializeModule()
         /// </summary>
         private static bool validationEnabled = true;
 
-
         /// <summary>
         /// Creates all possible attribute-based drawers and add them to proper collections.
         /// </summary>
         private static void PrepareAssignableDrawers(IToolboxInspectorSettings settings)
         {
             void AddAttributeDrawer<T>(Type drawerType, Type attributeType, Dictionary<Type, T> drawersCollection) where T : ToolboxAttributeDrawer
             {
-                if (drawerType == null)
+                if (drawerType == null || attributeType == null)
                 {
                     return;
                 }