unneeded popBacksStack simplified

app/src/main/java/de/ccc/events/badge/card10/filetransfer/FileTransferFragment.kt

@@ -175,7 +175,7 @@ class FileTransferFragment : Fragment(), GattListener, FileTransferListener{
                 .setMessage(message ?: getString(R.string.connection_error_generic))
                 .setPositiveButton(R.string.dialog_action_ok) {
                     dialog, _ -> dialog.dismiss()
-                    controller.popBackStack(R.id.mainFragment, false)
+                    controller.popBackStack()
                 }
                 .show()
     }