I pressed the shortcut to enter the lab (shift + B) and got this error message:
The mod The Blueprint Designer Lab (1.0.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event BlueprintLab_design::BPL_LabButtonHotkey (ID 169)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
BlueprintLab_design/scripts/gui.lua:29: in function 'UpdateGui'
BlueprintLab_design/scripts/logic.lua:15: in function 'OnLabButton'
BlueprintLab_design/scripts/gui.lua:55: in function <BlueprintLab_design/scripts/gui.lua:54>
stack traceback:
[C]: in function 'newindex'
__BlueprintLab_design/scripts/gui.lua:29: in function 'UpdateGui'
BlueprintLab_design/scripts/logic.lua:15: in function 'OnLabButton'
BlueprintLab_design/scripts/gui.lua:55: in function <BlueprintLab_design/scripts/gui.lua:54>

The lab button also completely disappeared after this, I can still load the save but I cant enter the lab.

Hey! Thanks for the report. I could not make the bug happen on my end. The keyboardshortcut works as intended. Maybe you culd provide me your savefile to test it? What other mods do you have enabled alongside this one?

I have a bunch of other ones Installed, but I have 2 save files of the same modpack, one has the lab while the other doesnt. Ill upload both, you should disable either endgame combat or darkstar they have a compatibility issue that I fixed myself. a2 is the bugged save file.
https://drive.google.com/drive/folders/17ydivDTrO3UPZPAvzwHDi6TyIiWmbdzr?usp=sharing

I had the same problem, and I found the issue. The reason this happens is the first line in InitPlayer (control.lua, line 30). If global[player_index] already exists, then the button in the top left corner never gets created, causing the error.

Steps to reproduce the crash: start a user scenario. Then press the keybind to enter the lab.

How to fix: remove the first line of InitPlayer . This check isn't necessary, because CreateGui calls playerData.flow.clear() before creating a new button.