Same issue, found your reply that claimed with notepad++ and line 996 my problem could be solved. Queue me trying to find the solution to my problem for 3 hours since the console text doesnt give me any hint, i don't know what file to edit, and hilariously enough you need to edit two different places, neither of which is 996 for me, i have deciphered your riddle Calrocka, and with it i have gained the ability to actually buy dungeon maps again, but holy hell would it have been faster if i knew where to start other than maybe 996 :(
Anyway, to fix this issue make a fresh download, open your fresh StriveConquest\src\core folder and find "input_handler.gd", this is the file causing issues. Open it in notepad++, open your modded versions "input_handler.gd" as well.
Ctrl+F and find 'location_purchase_event' and copy everything from it to scene.open(data) (which is at line 996 on the fresh install to Calrocka's credit)
Then find that same line in the modded version, it'll be around 970-990, and paste the fresh versions line over the modded versions line.
(Now if you were like me you'd start your game up and everything would break and crash and you'd try and figure out which lines are needed to keep or need to be replaced, do not be like me, be like Calrocka who just compared text lines to see what was different as i later did.)
Last Step, at lines 56-64~ you will find a bunch of lines starting with "var exploration_node" and ending with "var active_faction = {}"
However in the modded version it is missing "var selected_area" at line 61 between var active_location and var active_area
Copy var selected_area and paste it on line 61 so that your line up looks like this:
var exploration_node
var active_character
var scene_characters = []
var scene_loot
var active_area
var selected_area
var active_location
var selected_location
var active_faction = {}
Save the notepad++
now you can buy dungeons again :D