This happens when a mlc-gui is already open and I klick on another mlc to open that gui:

1625.079 Script @Moon_Logic/control.lua:214: {
x = 6,
y = 0
}
1927.868 Error MainLoop.cpp:1207: Exception at tick 3016319: Die Mod Moon Logic (0.0.13) hat einen Fehler verursacht, der nicht behoben werden kann.
Bitte informiere den Autor der Mod über diesen Fehler.

Error while running event Moon_Logic::on_gui_opened (ID 84)
Gui element with name mlc-gui already present in the parent element.
stack traceback:
Moon_Logic/gui.lua:113: in function 'elc'
Moon_Logic/gui.lua:119: in function 'create_gui'
Moon_Logic/gui.lua:220: in function 'open'
Moon_Logic/control.lua:515: in function <Moon_Logic/control.lua:504>
stack traceback:
[C]: in function 'add'
Moon_Logic/gui.lua:113: in function 'elc'
Moon_Logic/gui.lua:119: in function 'create_gui'
Moon_Logic/gui.lua:220: in function 'open'
Moon_Logic/control.lua:515: in function <Moon_Logic/control.lua:504>

Hm, yeah, code kinda supports opening multiple windows for these, but for me clickin on other combinator closes that window and opens a new one.
Not entirely sure if maybe it's some factorio option that makes them work like that, but dunno how to reproduce it.

Do it like constant combinator: only one gui can be open at the same time.

Yeah, probably should, as these guis are giant anyway.

Normally when you open a gui of an entity any other open entity-gui is closed. That is default behaviour.
In the current version of mlc any other gui remains open. Probably you should care for this too. But this is nice-to-have I guess.

In the current version of mlc any other gui remains open.

Yeah, weird, can't seem to reproduce it here with any two GUIs.
I.e. if I open chest and then click on MLC, chest one gets closed, MLC's opened, and vice-versa.

It happens only, when you first click into the edit area

It happens only, when you first click into the edit area

Ah, right, see that here as well, looks almost like a factorio bug :)

looks almost like a factorio bug

Ah, nope, that's the bug that I introduced by making "close gui" event on Esc key unfocus the editing area first, instead of closing whole gui.

Should be fixed in 0.0.14.