only show router disconnection warning when using external router

core/src/main/groovy/com/muwire/core/Core.groovy

@@ -162,7 +162,7 @@ public class Core {
     private final TrackerResponder trackerResponder
     final AutoUpdater autoUpdater
 
-    private final Router router
+    final Router router
 
     final AtomicBoolean shutdown = new AtomicBoolean()
     

gui/griffon-app/models/com/muwire/gui/MainFrameModel.groovy

@@ -806,6 +806,8 @@ class MainFrameModel {
     void onRouterDisconnectedEvent(RouterDisconnectedEvent e) {
         if (core.getShutdown().get())
             return
+        if (core.router != null)
+            return
         runInsideUIAsync {
             JOptionPane.showMessageDialog(null, trans("LOST_ROUTER_BODY"),
                 trans("LOST_ROUTER_TITLE"), JOptionPane.WARNING_MESSAGE)