Or you can follow the next steps in order to manually apply the fix
1. Open \Views\Catalog\ProductTemplate.SingleVariant.cshtml file
2. Replace
var defaultProductVariant = Model.ProductVariantModels.Count == 1 ? Model.ProductVariantModels[0] : null;
with
var defaultProductVariant = Model.ProductVariantModels.Count > 0 ? Model.ProductVariantModels[0] : null;