How to reproduce: run the attached project on a system with an increased scaling - 200%

Workaround: the null text needs to be set at run-time and after subscribe the control to the CreateTextBlock event

public class MyTextBlock : TextBlockElement
{
    public override void DpiScaleChanged(SizeF scaleFactor)
    {
        base.DpiScaleChanged(scaleFactor);
        this.ResetLayoutCore();
    }
} 

this.radTextBoxControl1.TextBoxElement.CreateTextBlock += this.RadTextBoxControl1_CreateTextBlock; 
private void RadTextBoxControl1_CreateTextBlock(object sender, CreateTextBlockEventArgs e)
{
    e.TextBlock = new MyTextBlock();
}