Remove some BUI boilerplate (#28399)

* Remove some BUI boilerplate

- The disposals overrides got removed due to the helper method handling it.
- Replace window creation with CreateWindow helper.
- Fixed some stinky code which would cause exceptions.

* More

* moar

* weh

* done

* More BUIs

* More updates

* weh

* moar

* look who it is

* weh

* merge

* weh

* fixes

Content.Client/Thief/ThiefBackpackBoundUserInterface.cs

@@ -1,6 +1,7 @@
 using Content.Shared.Thief;
 using JetBrains.Annotations;
 using Robust.Client.GameObjects;
+using Robust.Client.UserInterface;
 
 namespace Content.Client.Thief;
 
@@ -15,21 +16,9 @@ public sealed class ThiefBackpackBoundUserInterface : BoundUserInterface
     {
         base.Open();
 
-        _window = new ThiefBackpackMenu(this);
-        _window.OnClose += Close;
-        _window.OpenCentered();
-    }
-
-    protected override void Dispose(bool disposing)
-    {
-        base.Dispose(disposing);
-        if (!disposing)
-            return;
-
-        if (_window != null)
-            _window.OnClose -= Close;
-
-        _window?.Dispose();
+        _window = this.CreateWindow<ThiefBackpackMenu>();
+        _window.OnApprove += SendApprove;
+        _window.OnSetChange += SendChangeSelected;
     }
 
     protected override void UpdateState(BoundUserInterfaceState state)