In your code, is
x
null?
@Html.ValidationMessageFor(x => x.search_type)
It might be that the null object is being called for a property. Maybe check if there would be a validation message. Try this,
@if (!ViewData.ModelState.IsValid) {
}
This is one of the ways in which you are capturing the validation message, only if it exists. Otherwise, you ignore that statement at all.