fix: guard IsHandleCreated in EditNestForm timer

Prevent InvalidOperationException when the timer fires before or
after the control handle is available.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Forms/EditNestForm.cs

@@ -774,6 +774,9 @@ namespace OpenNest.Forms
 
         private void drawingListUpdateTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
         {
+            if (!drawingListBox1.IsHandleCreated)
+                return;
+
             drawingListBox1.Invoke(new MethodInvoker(() =>
             {
                 drawingListBox1.Refresh();