Numeric UpDown editors: fixed bug dealing with the FormatString I introduced when trying to fix another bug.

Main/Source/ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/NumericUpDown/Implementation/NumericUpDown.cs

@@ -39,6 +39,7 @@ namespace Microsoft.Windows.Controls
 
         protected virtual void OnFormatStringChanged(string oldValue, string newValue)
         {
+            if (IsInitialized)
                 Text = ConvertValueToText();
         }