To reproduce:
 private void MdiChildForm_Load(object sender, EventArgs e)
 {
      radMaskedEditBox1.MaskType = MaskType.Numeric;
      radMaskedEditBox1.Mask = "n4";
      radMaskedEditBox1.NullText = "this is null text"
}
 private void radButton1_Click(object sender, EventArgs e)
{
      radMaskedEditBox1.Value = null;
 }

the issue appears also when the RadmaskedEditBox is configured like this:

this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard;
this.radMaskedEditBox1.Mask = "0000000";

this.radMaskedEditBox1.NullText = "My null text";
this.radMaskedEditBox1.PromptChar = '_';

WORKAROUND:
radMaskedEditBox1.Value = null;
radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.HostedControl.Text = "";