Using unity's standard ObjectField in RequireReferenceDrawer to fix bugs and improve prefab workflow.

RequireReferenceAlert.cs

// PLEASE NOTE: This will add ~1sec to your save time
// If that is unacceptable (which it was to me), then you can change all the
// references to "MonoBehaviour" in this file to your own custom behaviour type
// that this script will search through each time you save

using UnityEditor;
using UnityEngine;
using System.Linq;
using System.Reflection;
using System.Collections;
using System.Collections.Generic;
using UnityEditor.Experimental.SceneManagement;

public class RequireReferenceAlert : UnityEditor.AssetModificationProcessor
{
    static string[] OnWillSaveAssets(string[] paths)
    {
        bool doValidate = false;
        foreach ( string path in paths )
        {
            if ( path.Contains( ".prefab" ) || path.Contains( ".scene" ) )
            {
                doValidate = true;
                break;
            }
        }

        if ( !doValidate )
            return paths;


        MonoBehaviour[] allBehaviours;
        PrefabStage stage = PrefabStageUtility.GetCurrentPrefabStage();
        if ( stage != null )
        {
            List< MonoBehaviour > foundBehaviours = new List< MonoBehaviour >();

            GameObject[] rootObjects = stage.scene.GetRootGameObjects();
            foreach ( GameObject rootObj in rootObjects )
                foundBehaviours.AddRange( rootObj.GetComponentsInChildren< MonoBehaviour >( true ) );

            allBehaviours = foundBehaviours.ToArray();
        }
        else
        {
            allBehaviours = Object.FindObjectsOfType< MonoBehaviour >();
        }

        foreach ( MonoBehaviour b in allBehaviours )
        {
            FieldInfo[] fields = b.GetType().GetFields();
            foreach ( FieldInfo field in fields )
            {
                object[] attrs = field.GetCustomAttributes( true );
                foreach ( object attr in attrs )
                {
                    if ( attr is RequireReferenceAttribute )
                    {
                        object obj = field.GetValue( b ) as object;
                        if ( obj is IEnumerable && obj.GetType().IsGenericType )
                        {
                            IEnumerable listObj = obj as IEnumerable;
                            foreach ( Object o in listObj )
                            {
                                if ( IsNullShowWarning( o, b.ToString(), b) )
                                    return new string[ 0 ];
                            }
                        }
                        else
                        {
                            if ( IsNullShowWarning( obj, b.ToString(), b ) )
                                return new string[ 0 ];
                        }
                        break;
                    }
                }
            }
        }

        return paths;
    }

    static bool IsNullShowWarning ( object obj, string name, Object selectObject )
    {
        if ( obj == null )
        {
            Selection.activeObject = selectObject;

            return !EditorUtility.DisplayDialog( 
                "Missing References",
                $"The following component is missing some required references: {name}",
                "Save Anyway",
                "Cancel"
            );
        }

        return false;
    }
} 

RequireReferenceAttribute.cs

public class RequireReferenceAttribute : PropertyAttribute
{
    public RequireReferenceAttribute ()
    {
        
    }
}

RequireReferenceDrawer.cs

using UnityEditor;
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System.Text.RegularExpressions;

[CustomPropertyDrawer(typeof(RequireReferenceAttribute))]
public class RequireReferenceDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        EditorGUI.BeginProperty(position, label, property);

        EditorGUI.ObjectField( position, property );

        if ( property.objectReferenceValue == null )
        {
            EditorGUI.DrawRect( position, new Color( 1.0f, 0.0f, 0.0f, 0.5f ) );
        }

        EditorGUI.EndProperty();
    }
}