Fix null reference crash in preview window

That's because MirrorData.Data could be null.

A property ``MirrorData.StringData`` is added to handle the string representation logic. 
Let me get it out of my system first, I have been wanting to say the following can be compacted to this:

```cs
public string StringData
{
    get
    {
        if (object.ReferenceEquals(Data, null) || this.IsNull)
            return ```null```;

        var str = Data.ToString();
        return ((Data is bool) ? str.ToLower() : str);
    }
}
```

Okay, that's just my personal preference, ignore me at will ;)