I just got this error in the latest version of factorio 18.9
I just clicked on the upgrade planner button and it happened

1299.279 Error MainLoop.cpp:1202: Exception at tick 235002: The mod Upgrade Builder and Planner (2.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event upgrade-planner-next::on_gui_click (ID 1)
Unknown sprite "utility/remove"
stack traceback:
upgrade-planner-next/upgrade-planner/gui.lua:133: in function 'open_frame'
upgrade-planner-next/upgrade-planner/gui.lua:277: in function 'handler'
stdlib/stdlib/event/event.lua:321: in function 'pcall'
stdlib/stdlib/event/event.lua:338: in function 'dispatch_event'
stdlib/stdlib/event/event.lua:424: in function <stdlib/stdlib/event/event.lua:375>
stack traceback:
[C]: in function 'add'
upgrade-planner-next/upgrade-planner/gui.lua:133: in function 'open_frame'
upgrade-planner-next/upgrade-planner/gui.lua:277: in function 'handler'
stdlib/stdlib/event/event.lua:321: in function 'pcall'
stdlib/stdlib/event/event.lua:338: in function 'dispatch_event'
stdlib/stdlib/event/event.lua:424: in function <stdlib/stdlib/event/event.lua:375>

Could you please verify you have the latest version (2.1.1) and try again?

yeah I just noticed that, and the error is the same as in the bug report for what you fixed. about to test this..
and the result is...
It is working now.
thanks and sorry for this duplicate report

No problem, glad it's working :)