Huh, looks like multiple problems can cause this same symptom, heh. In my case the error message is:
SCRIPT ERROR: farminspect: Invalid get index 'stallbedding' (on base: 'Dictionary').
At: res://files/scripts/Mansion.gd:3651
In fact, all the farm panel buttons are throwing a similar error:
SCRIPT ERROR: _on_vatsbutton_pressed: Invalid get index 'bottler' (on base: 'Dictionary').
At: res://files/scripts/Mansion.gd:5575
SCRIPT ERROR: _on_workersbutton_pressed: Invalid get index 'farmhand' (on base: 'Dictionary').
At: res://files/scripts/Mansion.gd:5846
SCRIPT ERROR: _on_storebutton_pressed: Invalid get index 'containers' (on base: 'Dictionary').
At: res://files/scripts/Mansion.gd:5902