Really simple utility to allow you to reference UnityEngine.Objects but only of a particular type as inspector fields

InterfaceObjectAttribute.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class InterfaceObjectAttribute : PropertyAttribute 
{
	public Type type;

	public InterfaceObjectAttribute(Type type){
		this.type = type;
	}
}

InterfaceObjectAttributeDrawer.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(InterfaceObjectAttribute))]
public class RangeDrawer : PropertyDrawer
{
    // Draw the property inside the given rect
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
		EditorGUI.ObjectField(position,property,label);
		//base.OnGUI(position,property,label);

		var obj = property.objectReferenceValue;
		if(obj!=null && !IsMatchingInterface(obj)){
			property.objectReferenceValue = null;
			EditorUtility.SetDirty(  property.serializedObject.targetObject );
		}

    }

	bool IsMatchingInterface(UnityEngine.Object obj){
		
		InterfaceObjectAttribute ioa = (InterfaceObjectAttribute)attribute;
		return ioa.type.IsAssignableFrom(obj.GetType());

	}
}