UnitySerializedReferenceUI/Editor/DefaultUI/SerializeReferenceInspectorButton.cs

#if UNITY_EDITOR
using System;
using System.Collections.Generic; 
using UnityEditor;
using UnityEngine;

namespace Textus.SerializeReferenceUI.Editor
{
    public static class SerializeReferenceInspectorButton
    {
        /// Must be drawn before DefaultProperty in order to receive input
        public static void DrawSelectionButtonForManagedReference(this SerializedProperty property, Rect position, IEnumerable<Func<Type, bool>> filters = null)
        {
            Rect buttonPosition = position;
            buttonPosition.x += EditorGUIUtility.labelWidth + 1 * EditorGUIUtility.standardVerticalSpacing;
            buttonPosition.width = position.width - EditorGUIUtility.labelWidth - 1 * EditorGUIUtility.standardVerticalSpacing;
            buttonPosition.height = EditorGUIUtility.singleLineHeight;

            Color storedColor = GUI.backgroundColor;
            int storedIndent = EditorGUI.indentLevel;
            EditorGUI.indentLevel = 0;

            (string AssemblyName, string ClassName) = ManagedReferenceUtility.GetSplitNamesFromTypename(property.managedReferenceFullTypename);

            bool isNull = string.IsNullOrEmpty(ClassName);
            GUI.backgroundColor = isNull ? Color.red : storedColor;

            string className = isNull ? "Null (Assign)" : ClassName;
            string assemblyName = AssemblyName;

            if (EditorGUI.DropdownButton(buttonPosition, new GUIContent(className, className + "  ( " + assemblyName + " )"), FocusType.Keyboard))
            {
                property.ShowContextMenuForManagedReference(buttonPosition, filters);
            }

            GUI.backgroundColor = storedColor;
            EditorGUI.indentLevel = storedIndent;
        }
    } 
}
#endif