While doing the hunt in the beastwarrens:

Message: Interface\AddOns\MythicPlusTimer\progress.lua:146: attempt to compare number with nil
Time: Mon Jan 18 18:40:41 2021
Count: 9
Stack: Interface\AddOns\MythicPlusTimer\progress.lua:146: attempt to compare number with nil
[string "@interface\AddOns\MythicPlusTimer\progress.lua"]:146: in function `callback'
[string "@interface\AddOns\MythicPlusTimer\addon.lua"]:100: in function <Interface\AddOns\MythicPlusTimer\addon.lua:94>

Locals: current_run = <table> {
is_completed = true
is_teeming = false
is_prideful = true
quantity_number = 500
cm_level = 13
elapsed_time = 2941
deathcount = 36
quantity_completed = true
time_left_3 = 0
zone_name = "Spires of Ascension"
affixes_key = "affixes-9-6-13-121"
time_left = 0
deathcount_visible = true
steps = 5
death_names = <table> {
}
current_zone_id = 2285
is_reaping = false
times = <table> {
}
max_time = 2340
affixes = <table> {
}
current_map_id = 381
pull = <table> {
}
level_key = "l13"
final_quantity_number = 500
time_left_2 = 0
}
_ = "Ritual Experimentation"
_ = "Defeat Displaced Soul Feeders before they are consumed by the Shifting Life-Eater."
steps = 1
_ = "Destroy Displaced Soul Feeders"
_ = 92
_ = false
_ = 9
final_value = 20
_ = 0
_ = 77829
quantity = "9"
quantity_number = nil
(*temporary) = 1
(*temporary) = ""
(*temporary) = nil
(*temporary) = ""
(*temporary) = 1
(*temporary) = false
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to compare number with nil"
main = <table> {
get_current_run = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:572
on_player_entering_world = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:496
enable = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:691
get_quest_frame = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:592
show_frame = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:655
show_default_tracker = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:643
get_frame = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:582
init = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:681
reset_current_run = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:577
show_demo = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:675
toggle_frame_movement = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:665
is_frame_moveable = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:670
hide_frame = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:660
name = "main"
get_info_frames = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:587
on_challenge_mode_start = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:509
is_in_cm = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:617
format_seconds = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:603
hide_default_tracker = <function> defined @interface\AddOns\MythicPlusTimer\main.lua:625
}
last_quantity = 1
last_kill = nil
update_progress_value = <function> defined @interface\AddOns\MythicPlusTimer\progress.lua:20

Should no longer happen with the latest version