self.oldfeeRate is initialized as int in `QETxRbfFeeBumper` and
`QETxCanceller`. However QML expects it to be a string. Initializing
it as string fixes the exception. Previously this didn't happen as
`Abstract_Wallet.add_info_from_wallet_and_network()` would never
return False, so the `get_tx()` method would immediately overwrite
the self.oldfeeRate variable with a string.
```
20.95 | D | gui.qml.qetxfinalizer | TxMonMixin.__init__
20.95 | E | gui.qml.qeapp.Exception_Hook | exception caught by crash reporter
TypeError: unable to convert a Python 'int' object to a C++ 'QString' instance
```
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