Set or Transmit Atrributes generates an error when used with "Replace" existing code
#### Summary:
When I use Set_Atrributes extension, if I choose to replace an existing Javascript code linked to an event the extension generates an error.
#### Steps to reproduce:
- Start Inkscape.
- Create two shapes then select them both.
- Go to Extensions>Web>JavaScript>Set_Attributes.
- In Options tab set "When should the set be done" to "on click", then set "Compatibility with previous code to this event" to "Run it after", then click apply.
- In "value to set" change the color (e.g green), then set "Compatibility with previous code to this event" to "Replace", then click on Apply.
#### What happened?
The following error is shown:
```
Traceback (most recent call last):
File "web_set_att.py", line 69, in <module>
SetAttribute().run()
File "/usr/share/inkscape/extensions/inkex/base.py", line 123, in run
self.save_raw(self.effect())
File "web_set_att.py", line 66, in effect
elem.set(self.options.when, el_ev_code)
UnboundLocalError: local variable 'el_ev_code' referenced before assignment
```
#### What should have happened?
New color should be set without error.
#### NB:
The same bug exists with Extensions>Web>JavaScript>Transmit_Attributes extension.
#### Attachments:
#### Version Info:
```
Inkscape Version: 1.0 (4035a4fb49, 2020-05-01)
Operating System: Manjaro Linux
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.15.0
Kernel Version: 5.7.0-3-MANJARO
OS Type: 64-bit
```
issue