Fixes: #7794

Marked the Model property on the DataGridRowValidator as virtual so, that it can be overriden. Existing DatagridRowValidator.Model throws the NotImplementedexception when accessed. See open discussion for more details: #7767

Description

I would like to use the built in mudblazor validation (For) in the MudDataGrid in combination with the FluentValidator. That could potentially be done quite nicely in a similar clean way as it's already possible with other controls like MudTextField.

Here is the problem:
MudDataGrid.razor.cs contains the validator property:
public Interfaces.IForm Validator { get; set; } = new DataGridRowValidator();

There is currently no working implementation in place for the DataGrid and trying to use the mentioned validator leads to a crash because of the following implementation in the DataGridRowValidator :
public object? Model { get => throw new System.NotImplementedException(); set => throw new System.NotImplementedException(); }

The MudFormComponent would start the validation using "For" and crashes because of Form?.Model

protected virtual async Task ValidateModelWithFullPathOfMember(Func<object, string, Task<IEnumerable<string?>>> func, List<string> errors)
{
    try
    {
        if (Form?.Model is null)
        {
            return;
        }

The implementation is just not in place yet, which is perfectly fine.
Unfortunately it is currently impossible to extend or override the functionality because of the Model Property implementation (or rather the fact that it's missing and throws an exception) on the DataGridRowValidator. It's also impossible to provide a completely new implementation for the IForm since some of te properties are marked as internal.

    public interface IForm
    {
        public bool IsValid { get; }
        public string[] Errors { get; }
#nullable enable
        public object? Model { get; set; }
        public void FieldChanged(IFormComponent formControl, object? newValue);
#nullable disable
        internal void Add(IFormComponent formControl);
        internal void Remove(IFormComponent formControl);
        internal void Update(IFormComponent formControl);
    }

The easiest solution would probably be to mark the Model property on the DataGridRowValidator as virtual
The alternative would be to allow setting of Model on DataGridRowValidator. If you would prefer that let me know and I'll provide a new PR.

How Has This Been Tested?

I've created a small application to check whether the change would suffice for what I was intending to do. (override model and provide my own implementation. Honestly a normal get/set instead of an exception would have probably already be enough but I just wanted to have as little of an impact as possible.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.