Fix compilation errors for Unity 2019

Author: arimger

Assets/Editor Toolbox/Editor/Utilities/PropertyUtility.cs

@@ -428,7 +428,7 @@ public static void OverrideLabelByValue(GUIContent label, SerializedProperty pro
                 case SerializedPropertyType.ObjectReference:
                     label.text = property.objectReferenceValue ? property.objectReferenceValue.ToString() : "null";
                     break;
-#if UNITY_2019_3_OR_NEWER
+#if UNITY_2021_3_OR_NEWER
                 case SerializedPropertyType.ManagedReference:
                     label.text = property.managedReferenceValue?.ToString() ?? "null";
                     break;