Here's some extra information--looking in the source code
gnuradio/gnuradio-runtime/lib/tagged_stream_block.cc, this block of code is specifically what seems to cause the error message.
Specifically, I'm wondering what Line 117 does, because it looks suspiciously like it's setting d_n_input_items_reqd= -1 based on something input_items.size(), which would cause a runtime error in the next for-loop, which matches the error I get exactly. I tried to find out what ".assign" does but I don't know where to look for that, because I don't know what sort of thing d_n_input_items_reqd is, and I haven't been successful in finding "tagged_stream_block.h", in github.
^If anyone knows about this, please let me know!
There's nowhere else I suspect where the value of d_n_input_items_reqd changes from its initial value of input_signature->min_streams()
I'm not anything close to a software engineer being a software engineer, so if anyone who knows this code can help me out, that'd be greatly appreciated. Apologies if I've said anything that's blatantly wrong...