NavigationForm: fix crash on re-opening

- maybe there's better ways but to fix we intercept Close() events, cancel, and Hide() instead
- fixes #92

DiztinGUIsh/window/NavigationForm.cs

@@ -34,6 +34,16 @@ public ISnesNavigation SnesNavigation
         public NavigationForm()
         {
             InitializeComponent();
+            FormClosing += Navigation_Closing;
+        }
+
+        private void Navigation_Closing(object sender, FormClosingEventArgs e)
+        {
+            if (e.CloseReason != CloseReason.UserClosing) 
+                return;
+
+            e.Cancel = true;
+            Hide();
         }
 
         private void Navigation_Load(object sender, EventArgs e)