fix: check validity of node instance before accessing it.

addons/GodotDI/Editor/Plugin.cs

@@ -77,6 +77,9 @@ public partial class Plugin : EditorPlugin
         // mark debounce as completed
         _sceneTreeTimer = null;
 
+        // check that the instance is still valid after debouncing (could have been disposed)
+        if (!IsInstanceValid(sceneRoot)) return;
+
         var sceneNodes = sceneRoot.FindChildren("*").Where(node => node.Owner == sceneRoot).Append(sceneRoot).ToList();
         GD.Print($"Updating injection for root {sceneRoot.Name}");