abort export privkeys if wallet has no seed

gui/qt/main_window.py

@@ -1922,6 +1922,10 @@ class ElectrumWindow(QMainWindow):
 
     @protected
     def do_export_privkeys(self, password):
+        if not self.wallet.seed:
+            self.show_message(_("This wallet has no seed"))
+            return
+
         self.show_message("%s\n%s\n%s" % (_("WARNING: ALL your private keys are secret."),  _("Exposing a single private key can compromise your entire wallet!"), _("In particular, DO NOT use 'redeem private key' services proposed by third parties.")))
 
         try: