Maverik will probably release a fixed version sometime soon. I have a bugfix posted on the discord, in the mods channel. https://itch.io/t/284398/discord
To quick fix this issue yourself, open globals.gd in a decent text editor and add these lines after line 1278:
if !state.mansionupgrades.has("farmmana"):
state.mansionupgrades.farmmana = 0
First line should have 1 tab, second line should have 2 tabs.