Fixed NRE when node search's category color is reset

Jira: UUM-71376

Steps to reproduce:
1. Create project using High Definition 3D Template
2. Create VFX Asset and open it
3. In New VFX window press space or right-click to open VFX Node Search Window
4. In VFX Node Search window press on “>” to show details panel
5. Select the “Context” folder and change the folder color
6. Once the color of the folder is changed press “Reset color to default” button
7. Observe the Console window

Actual results: "NullReferenceException: Object reference not set to an instance of an object" error is thrown when pressing reset button
Expected results: No errors when pressing reset button

Reproducible with versions: 6000.0.1f1
Not reproducible with versions: 6000.0.0f1

Tested on (OS): macOS Sonoma 14.4.1 (M1), Windows 10

![Video](https://jira.unity3d.com/secure/attachment/1443182/Screen%20Recording%202024-05-03%20at%2010.51.02.mov)

Author: julienamsellem

Packages/com.unity.visualeffectgraph/Editor/FilterPopup/VFXFilterWindow.cs

@@ -318,7 +318,7 @@ private void OnResetCategoryColor(ClickEvent evt)
             var descriptor = (Descriptor)m_Treeview.selectedItem;
             settings.ResetCategoryColor(descriptor.name);
             var element = m_Treeview.GetRootElementForIndex(m_Treeview.selectedIndex);
-            var label = element.Q<Label>("descriptorLabel");
+            var label = element.Q<Label>("categoryLabel");
             label.style.unityBackgroundImageTintColor = new StyleColor(StyleKeyword.Null);
             m_CategoryColorField.value = label.resolvedStyle.unityBackgroundImageTintColor;
         }