For what it's worth, selecting a dungeon after leaving it, but not forgetting it, gives the following error and kicks you to the main menu:
SCRIPT ERROR: select_location: Invalid get index 'id' (on base: 'Nil').
At: res://gui_modules/Universal/Scripts/NavigationModule.gd:112.
This is on 0.5.3a, (Linux v20). I've seen worse behaviour like a grey screen I could not get out of, when my party left a bandit den I did tell the game to forget, and then clicked on the den location along the top. But I wasn't watching the console beforehand and some of the errors might have been there already:SCRIPT ERROR: select_location: Invalid get index 'id' (on base: 'Nil').
At: res://gui_modules/Universal/Scripts/NavigationModule.gd:112.
SCRIPT ERROR: get_location_characters: Invalid get index 'id' (on base: 'Nil').
At: res://src/core/input_handler.gd:1352.
SCRIPT ERROR: build_location_group: Unable to iterate on object of type 'Nil'.
At: res://gui_modules/Exploration/Scripts/ExplorationModule.gd:858.
SCRIPT ERROR: _input: Invalid get index 'type' (on base: 'Nil').
At: res://src/core/input_handler.gd:403.
SCRIPT ERROR: get_location_characters: Invalid get index 'id' (on base: 'Nil').
At: res://src/core/input_handler.gd:1352.
SCRIPT ERROR: build_location_group: Unable to iterate on object of type 'Nil'.
At: res://gui_modules/Exploration/Scripts/ExplorationModule.gd:858.
SCRIPT ERROR: get_location_characters: Invalid get index 'id' (on base: 'Nil').
At: res://src/core/input_handler.gd:1352.
SCRIPT ERROR: build_location_group: Unable to iterate on object of type 'Nil'.
At: res://gui_modules/Exploration/Scripts/ExplorationModule.gd:858.
SCRIPT ERROR: get_location_characters: Invalid get index 'id' (on base: 'Nil').
At: res://src/core/input_handler.gd:1352.
SCRIPT ERROR: build_location_group: Unable to iterate on object of type 'Nil'.
At: res://gui_modules/Exploration/Scripts/ExplorationModule.gd:858.
SCRIPT ERROR: _input: Invalid get index 'type' (on base: 'Nil').
At: res://src/core/input_handler.gd:403.