NetworkMenu: destroy the wifi dialog when the associate device disappears

When the device is removed by NetworkManager, we can't use it
to connect, so the dialog is useless and should be destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=723935

js/ui/status/network.js

@@ -1170,6 +1170,10 @@ const NMDeviceWireless = new Lang.Class({
             this._client.disconnect(this._wirelessHwEnabledChangedId);
             this._wirelessHwEnabledChangedId = 0;
         }
+        if (this._dialog) {
+            this._dialog.destroy();
+            this._dialog = null;
+        }
 
         this.item.destroy();
     },