fix: check that we're accessing the tree only when possible.

addons/GodotDI/Editor/Plugin.cs

@@ -1,8 +1,9 @@
 #if TOOLS
+using System;
 using System.Linq;
 using System.Reflection;
 using Godot;
-using Godot.Collections;
+using Array = Godot.Collections.Array;
 
 namespace GodotHostTest.GodotDI.Editor;
 
@@ -18,9 +19,10 @@ public partial class Plugin : EditorPlugin
         BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
 
     private ScopeInspectorPlugin? _plugin;
-
     private SceneTreeTimer? _sceneTreeTimer;
 
+    private readonly object _sceneTreeTimerLock = new();
+
     private bool _treeChangesRegistered;
 
     public override void _EnterTree()
@@ -54,7 +56,7 @@ public partial class Plugin : EditorPlugin
         OnSceneChanged(root);
     }
 
-    private void OnSceneChanged(Node? sceneRoot)
+    private async void OnSceneChanged(Node? sceneRoot)
     {
         // register to tree changes, in case the registration wasn't possible before
         RegisterTreeChanges();
@@ -63,16 +65,26 @@ public partial class Plugin : EditorPlugin
         if (sceneRoot == null) return;
 
         // apply debounce
+        var editedSceneRoot = EditorInterface.Singleton.GetEditedSceneRoot();
+
+        if (!editedSceneRoot.IsInsideTree()) await ToSignal(editedSceneRoot, Node.SignalName.TreeEntered);
+
+        var tree = editedSceneRoot.GetTree();
+        if (tree == null)
+            throw new InvalidOperationException("editedSceneRoot.GetTree() was null even after TreeEntered");
+
+        lock (_sceneTreeTimerLock)
+        {
             if (_sceneTreeTimer != null)
             {
                 _sceneTreeTimer.SetTimeLeft(DebounceDelay);
                 return;
             }
 
-        _sceneTreeTimer =
-            EditorInterface.Singleton.GetEditedSceneRoot().GetTree().CreateTimer(DebounceDelay);
+            _sceneTreeTimer = tree.CreateTimer(DebounceDelay);
             _sceneTreeTimer.Timeout += () => ActualOnSceneChanged(sceneRoot);
         }
+    }
 
     private void ActualOnSceneChanged(Node sceneRoot)
     {