Stop using it and treat the smell as a safety concern. Do not keep running to see whether it clears or assume it is ordinary food residue.
Understanding this situation
Use safe isolation where possible and get out, stay out and call 999 from a safe place if smoke or fire is present. A recently introduced plastic item is relevant history but does not prove that the appliance itself is unaffected.
A useful next step
Leave the machine unused pending appropriate advice. Report the programme stage, load and any electrical symptoms. Do not dismantle controls or run an empty hot cycle to burn away a presumed residue.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.