Hard crash when exiting sandbox. I dont know how to replicate, but its happened to me twice after AFKing in a sandbox for a VERY long time. Will post this under both mods.

The mod Blueprint Sandboxes (2.1.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event blueprint-sandboxes::on_lua_shortcut (ID 37)
The mod miniMAXIme: Character scaler and selector (2.0.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event minime::on_player_changed_force (ID 60)
bool expected, got nil
stack traceback:
[C]: in function 'newindex'
__minime/scripts/gui_stuff/gui_available_chars.lua:901: in function 'unhide_gui'
minime/scripts/gui.lua:208: in function 'enable_guis'
minime/scripts/forces.lua:280: in function 'event_handler'
minime/scripts/events.lua:288: in function <minime/scripts/events.lua:285>
stack traceback:
[C]: in function 'set_controller'
blueprint-sandboxes/scripts/sandbox.lua:277: in function 'RecoverPlayerCharacter'
blueprint-sandboxes/scripts/sandbox.lua:228: in function 'Exit'
blueprint-sandboxes/scripts/sandbox.lua:440: in function 'Toggle'
blueprint-sandboxes/scripts/toggle-gui.lua:231: in function <blueprint-sandboxes/scripts/toggle-gui.lua:229>

Thanks for the report! However, this looks like a normal crash where closing the error message takes you back to the main menu. I consider a hard crash one that triggers an error in Factorio (not a mod), so the entire game quits and you have to restart before you get to the main menu.

Anyway, I'll look into this later today …

Do you still have a saved game from when you were in the sandbox? If so, could you make the following change to scripts/gui_stuff/gui_available_chars.lua, reload that saved game, and return from the sandbox?

Replace the block from line 899 to line 907:

        gui.tags = tags
        if tags ~= nil then
          gui.visible = tags.visible
        else
          gui.visible = true
        end
minime.show("gui.visible", gui.visible)

        tags.visible = nil

with this:

        if tags.visible ~= nil then
          gui.visible = tags.visible
        else
          gui.visible = true
          tags.visible = true
        end

Still crashed, but I think the player character still exists. The amour slot was empty before when it crashed, now the regular amour is there. So progress.

The mod Blueprint Sandboxes (2.1.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event blueprint-sandboxes::on_lua_shortcut (ID 37)
The mod miniMAXIme: Character scaler and selector (2.0.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event minime::on_player_changed_force (ID 60)
bool expected, got nil
stack traceback:
[C]: in function 'newindex'
__minime/scripts/gui_stuff/gui_char_selector.lua:1278: in function 'unhide_gui'
minime/scripts/gui.lua:208: in function 'enable_guis'
minime/scripts/forces.lua:280: in function 'event_handler'
minime/scripts/events.lua:288: in function <minime/scripts/events.lua:285>
stack traceback:
[C]: in function 'set_controller'
blueprint-sandboxes/scripts/sandbox.lua:277: in function 'RecoverPlayerCharacter'
blueprint-sandboxes/scripts/sandbox.lua:228: in function 'Exit'
blueprint-sandboxes/scripts/sandbox.lua:440: in function 'Toggle'
blueprint-sandboxes/scripts/toggle-gui.lua:231: in function <blueprint-sandboxes/scripts/toggle-gui.lua:229>

Thanks! I was a bit confused because the erroneous code still seemed to be in there – but it was in the unhide function for the other GUI. Should be fixed in version 2.0.5! :-)

He lives. Thank you so much.

You're welcome, glad it's working again! :-)