Fix set import shortly after starting program

CB_Form is not yet initialized with items, do a little sanity checking
before setting index to 0.
Closes #729

PKHeX.WinForms/MainWindow/Main.cs

@@ -525,7 +525,7 @@ namespace PKHeX.WinForms
             for (int i = 0; i < formStrings.Length; i++)
                 if (formStrings[i].Contains(Set.Form ?? ""))
                 { form = i; break; }
-            CB_Form.SelectedIndex = form;
+            CB_Form.SelectedIndex = Math.Min(CB_Form.Items.Count-1, form);
 
             // Set Ability
             int[] abilities = SAV.Personal.getAbilities(Set.Species, form);