[UUM-133526] Material Editor - fix warning from GUILayout (#659)

* Add missing End GUILayout calls

* Add changelog entry

* Use ExitGUI() to leave the IMGUI scope early

Author: thomas-tu

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - [UUM-133861] Fixed "Look rotation viewing vector is zero" log being spammed when holding shift while using a create tool such as Create Sprite.
+- [UUM-133526] Material Editor: fixed a warning (`GUI Error: Invalid GUILayout state in MaterialEditor view.`) that was thrown when deleting an extra material slot.
 - Fixed warnings related to obsolete API calls with Unity 6.4 and onwards.
 
 ## [6.0.9] - 2026-01-30

Editor/EditorCore/MaterialEditor.cs

@@ -361,6 +361,8 @@ void OnGUI()
                         System.Array.Copy(materials, 0, temp, 0, materials.Length - 1);
                         materials = temp;
                         SaveUserMaterials(materials);
+
+                        GUIUtility.ExitGUI();
                         return;
                     }
                     GUI.backgroundColor = Color.white;