Please report this error to the mod author.

Error while running event BeastFinder::on_gui_click (ID 1)
BeastFinder/scripts/BeastFinder-control.lua:848: table index is nil
stack traceback:
BeastFinder/scripts/BeastFinder-control.lua:848: in function 'FindMatches'
BeastFinder/scripts/BeastFinder-control.lua:1063: in function 'DoSearch'
BeastFinder/scripts/BeastFinder-control.lua:1364: in function <BeastFinder/scripts/BeastFinder-control.lua:1253>
Please report this error to the mod author.

Error while running event BeastFinder::on_gui_click (ID 1)
BeastFinder/scripts/BeastFinder-control.lua:848: table index is nil
stack traceback:
BeastFinder/scripts/BeastFinder-control.lua:848: in function 'FindMatches'
BeastFinder/scripts/BeastFinder-control.lua:1063: in function 'DoSearch'
BeastFinder/scripts/BeastFinder-control.lua:1364: in function <BeastFinder/scripts/BeastFinder-control.lua:1253>"

Thanks for report.
I need some more detail to understand why this is happening... can you repeat the error?
Was it in multiplayer?

If the issue persists, it may be worth trying to reset your settings...
Console command to reset to default settings:
/c remote.call("BeastFinder", "Reset")

I forgot it

No worries. If it happens again take a note of what you were searching for etc.

maybe is a unkonw item

Also, it would be helpful if you send a copy of your save file and the log file, if you are okay with that. The FAQ contains a short list of things you can provide that might be helpful for debugging.

maybe is a unkonw item

Sounds possible... if you search for a modded item, then disable the mod, the now unknown item may still be in the search items list when you reload.