Hi!
I've faced the following error while finishing the research "Minining Productivity 2":

The mod Spaceblock (0.3.7) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event spaceblock::on_research_finished (ID 18)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
spaceblock/control.lua:83: in function 'RefreshAssemblers'
spaceblock/control.lua:106: in function 'y'
spaceblock/lib/lib_control.lua:284: in function <spaceblock/lib/lib_control.lua:284>
stack traceback:
[C]: in function 'index'
__spaceblock/control.lua:83: in function 'RefreshAssemblers'
spaceblock/control.lua:106: in function 'y'
spaceblock/lib/lib_control.lua:284: in function <spaceblock/lib/lib_control.lua:284>

Thanks for report, i'll be looking into this over the next day or two.

still issue also causing memory leak. Haven't had good opportunity to get to this.

Hello. I wanted to report an issue with Mining Productivity 4, that seems connected to this.
(Mining Productivity 2 was finished without issues interestingly enough)
Here is the error:

The mod Spaceblock (0.3.7) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event spaceblock::on_research_finished (ID 18)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
spaceblock/control.lua:83: in function 'RefreshAssemblers'
spaceblock/control.lua:106: in function 'y'
spaceblock/lib/lib_control.lua:284: in function <spaceblock/lib/lib_control.lua:284>
stack traceback:
[C]: in function 'index'
__spaceblock/control.lua:83: in function 'RefreshAssemblers'
spaceblock/control.lua:106: in function 'y'
spaceblock/lib/lib_control.lua:284: in function <spaceblock/lib/lib_control.lua:284>

Thank you for looking into this! (When you are able to)
Also, if you have any ideas of a workaround, please let us know.
(Perhaps if we stopped the assemblers, downgraded them, removed modules, or similar. I don't really have enough knowledge of the internals to know.)

I have the same problem. I tried to fix it by unlocking a higher Mining Productivity with the editor directly but it still crashes. Mining Productivity 1 reasearch worked fine but anything beyond that crashes the game.