remove static instance field

src/SharpIDE.Godot/IdeRoot.cs

@@ -48,7 +48,6 @@ public partial class IdeRoot : Control
 		GodotGlobalEvents.Instance = new GodotGlobalEvents();
 		GlobalEvents.Instance = new GlobalEvents();
 		BuildService.Instance = new BuildService(); // TODO: Sort out this mess with singletons, especially access across Application services
-		IdeOpenTabsFileManager.Instance = new IdeOpenTabsFileManager();
 	}
 
 	public override void _ExitTree()

src/SharpIDE.Godot/IdeWindow.cs

@@ -47,6 +47,7 @@ public partial class IdeWindow : Control
         // PrintOrphanNodes();
     }
     
+    // TODO: Problematic, as this is called even when the focus shifts to an embedded subwindow, such as a popup 
     private void OnFocusExited()
     {
         _ = Task.GodotRun(async () => await _openTabsFileManager.SaveAllOpenFilesAsync());