Restric ReferencePicker to Unity 2019.3 or higher

Author: arimger

Assets/Editor Toolbox/Editor/Drawers/Toolbox/PropertySelf/ReferencePickerAttributeDrawer.cs

@@ -1,4 +1,5 @@
-using System;
+#if UNITY_2019_3_OR_NEWER
+using System;
 
 using UnityEditor;
 using UnityEngine;
@@ -77,4 +78,5 @@ public override bool IsPropertyValid(SerializedProperty property)
             return property.propertyType == SerializedPropertyType.ManagedReference;
         }
     }
-}
+}
+#endif

Assets/Editor Toolbox/Scripts/Attributes/Toolbox/PropertySelfAttributes/ReferencePickerAttribute.cs

@@ -1,4 +1,5 @@
-using System;
+#if UNITY_2019_3_OR_NEWER
+using System;
 using System.Diagnostics;
 
 namespace UnityEngine
@@ -12,4 +13,5 @@ namespace UnityEngine
     [Conditional("UNITY_EDITOR")]
     public class ReferencePickerAttribute : ToolboxSelfPropertyAttribute
     { }
-}
+}
+#endif