Replaced port area helper and avoided crash when searching at a harbour that has no entries

ENI2/DetailViewControls/PortNotificationDetailControl.xaml.cs

@@ -522,6 +522,8 @@ namespace ENI2.DetailViewControls
         }
 
         private void buttonSearchPortArea_Click(object sender, RoutedEventArgs e)
+        {
+            if (portAreas != null)
             {
                 SelectPortAreaDialog spad = new SelectPortAreaDialog(this.Core.PoC);
                 if (spad.ShowDialog() ?? false)
@@ -536,6 +538,7 @@ namespace ENI2.DetailViewControls
                     }
                 }
             }
+        }
 
         #endregion