qt installwizard: fix empty filename crash

Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\installwizard.py", line 256, in on_filename
    widget_create_new.setVisible(temp_storage and temp_storage.file_exists())
TypeError: setVisible(self, bool): argument 1 has unexpected type 'NoneType'

electrum/gui/qt/installwizard.py

@@ -253,7 +253,7 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
             if msg is None:
                 msg = _('Cannot read file')
             self.msg_label.setText(msg)
-            widget_create_new.setVisible(temp_storage and temp_storage.file_exists())
+            widget_create_new.setVisible(bool(temp_storage and temp_storage.file_exists()))
             if user_needs_to_enter_password:
                 self.pw_label.show()
                 self.pw_e.show()