I've run into this repeatedly throughout several versions or both Blueprint Sanboxes and Miniloader-redux, Using the Alt+U shortcut for the upgrader tool, upgrading a miniloader causes an instant error, this only happens in the Blueprint Sandbox. Edit: Placing a different speed loader e.g. a red one over a yellow one, also gives the same error. I had this issue in my playthrough and then later tested with only these two mods enabled, both in space age and base game and it gives the same error everytime (see below):

The mod Blueprint Sandboxes (3.2.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event blueprint-sandboxes::on_marked_for_upgrade (ID 136)
The mod Miniloader (Redux) (0.11.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event miniloader-redux::script_raised_built (ID 92)
miniloader-redux/scripts/controller.lua:172: assertion failed!
stack traceback:
[C]: in function 'assert'
miniloader-redux/scripts/controller.lua:172: in function 'create_loader'
miniloader-redux/scripts/controller.lua:316: in function 'setup'
miniloader-redux/scripts/controller.lua:341: in function 'create'
miniloader-redux/scripts/event-setup.lua:73: in function 'handler'
miniloader-redux/stdlib/event/event.lua:353: in function 'pcall'
miniloader-redux/stdlib/event/event.lua:370: in function 'dispatch_event'
miniloader-redux/stdlib/event/event.lua:449: in function <miniloader-redux/stdlib/event/event.lua:407>
stack traceback:
[C]: in function 'create_entity'
blueprint-sandboxes/scripts/god.lua:160: in function 'handler'
blueprint-sandboxes/scripts/god.lua:216: in function 'AsyncWrapper'
blueprint-sandboxes/scripts/god.lua:277: in function <blueprint-sandboxes/scripts/god.lua:275>