Export plot extension returns error message "The output from the extension could not be parsed"

Steps to reproduce:

• open Inkscape
• create a simple drawing - for example, a single square
• highlight object and turn object to path
• highlight object and select extensions, export, plot
• send drawing to plotter/cutter using the HPGL option

What happened?

Inkscape cut the required shape but then returned the error message "The output from the extension could not be parsed".

What should have happened?

There should not have been an error message.

Inkscape Version and Operating System:

• Inkscape Version:1.0 beta 2 (2b71d25, 2019-12-03)
• Operating System: Ubuntu
• Operating System version: 18.04.3 LTS
• Plotter/cutter used - Liyu SC631 cutter, accessed through USB port

I understand the relevant line in the extension reads: return init + hpgl + ';SP0;PU0,0;@ '. I know nothing about HPGL but have not seen the @ command in any of the guides on the internet to HPGL commands so I wonder if this is the problem. The equivalent line, line 107, in plotter.py used with Inkscape 0.92, which does not return the parsing error reads: self.hpgl = hpglInit + self.hpgl + ';SP0;PU0,0;IN; '. I wonder therefore if the solution is to replace @ with IN?

Note that as mentioned in Issue 1479 this line probably needs to be edited to change the order of the SP0 and PU0,0 commands to solve the separate "cutting unwanted extra line" bug.