transaction_dialog: hide broadcast button when offline

gui/qt/transaction_dialog.py

@@ -178,6 +178,7 @@ class TxDialog(QDialog):
 
         # if we are not synchronized, we cannot tell
         if self.parent.network is None or not self.parent.network.is_running() or not self.parent.network.is_connected():
+            self.broadcast_button.hide()  # cannot broadcast when offline
             return
         if not self.wallet.up_to_date:
             return