Entities: Use 'required' and '= null!' to get rid of warnings

Elwig/Windows/BaseDataWindow.xaml.Mod.cs

@@ -103,7 +103,9 @@ namespace Elwig.Windows {
             _modChanged = true;
             var idx = (SeasonModifierList.SelectedIndex != -1) ? SeasonModifierList.SelectedIndex + 1 : _modList.Count;
             var item = new Modifier {
-                Year = s.Year
+                Year = s.Year,
+                ModId = "",
+                Name = "",
             };
             _modList.Insert(idx, item);
             SeasonModifierList.SelectedIndex = idx;