Most of the times weird issues like this are caused by the code calling the property. Can you paste a snipped of the code that refers to this model?

I figured it out!!! The issue is that your content model is missing a fallback.

Here is a simplified example from within a Home Page RenderController using Umbraco 12:

using ContentModels = Umbraco.Cms.Web.Common.PublishedModels;

public class HomeController : RenderController
 {
     private IPublishedValueFallback _publishedValueFallback;
     private readonly ServiceContext _serviceContext;
     private readonly IVariationContextAccessor _variationContextAccessor;


     public HomeController(
             ServiceContext context, 
             ICompositeViewEngine compositeViewEngine,
             IUmbracoContextAccessor umbracoContextAccessor,
             IPublishedValueFallback publishedValueFallback, 
             IVariationContextAccessor variationContextAccessor
          )
         : base(compositeViewEngine, umbracoContextAccessor)
         {
             _publishedValueFallback = publishedValueFallback;
             _serviceContext = context;
             _variationContextAccessor = variationContextAccessor;
         }

     public override IActionResult Index()
     {
         //This is the normal method of including a fallback.
         //In this case, we are using the injected fallback since this model is related to the current page coming in.
         var cmHome = new ContentModels.Home(CurrentPage, _publishedValueFallback);


         //Loop through the child nodes:
         foreach (IPublishedContent ipChild in cmHome.Children)
         {
            //Here we want to instantiate a model different from the parent. 
            //In this case we want to create a new fallback.
            ContentModels.ChildPg cmChild = new ContentModels.ChildPg(ipChild, new PublishedValueFallback(_serviceContext, _variationContextAccessor));

            //Now you can check for null without having a NullReferenceException being thrown.
            if (cmChild.myProperty != null)
            {
                //Get the values
            }
         }
    ...

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