fix fee slider offline bug

gui/qt/main_window.py

@@ -905,6 +905,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
             from electrum.util import fee_levels
             i = self.fee_slider.sliderPosition()
             tooltip = fee_levels[i]
+            if self.network:
                 dynfee = self.network.dynfee(i)
                 if dynfee:
                     tooltip += '\n' + self.format_amount(dynfee) + ' ' + self.base_unit() + '/kB'