Prevent crash when defining user variants without selected index.

src/Winp/Form/ServiceForm.cs

@@ -223,7 +223,7 @@ private ServiceReference CreateServiceReference(IPackage package, ServiceRunner?
         foreach (var variant in variants)
             variantComboBox.Items.Add(variant);
 
-        variantComboBox.SelectedIndex = currentVariantIndex + indexOffset;
+        variantComboBox.SelectedIndex = Math.Min(currentVariantIndex + indexOffset, variantComboBox.Items.Count - 1);
         variantComboBox.SelectedIndexChanged += (_, _) =>
         {
             setVariantIndex(variantComboBox.SelectedIndex - indexOffset);