A common protocol structure is to have a header containing a length field, followed by data of length bytes, followed by additional protocol fields such as a CRC. If the length field is variable, the position of subsequent URH-specified protocol fields defined by fixed position will not be in the correct location.

I do not know the best way to address this, but quick and easy way to support

Just a little annoyance if you have to kill GRC while it's executing a flowgraph the recent opened flowgraphs is reset to the last time gnuradio-companion was closed properly. From a UI point of view it would be nice to have it restored to the last possible moment and saving the recently opened graphs before running something would seem reasonable from a UX perspective.