Sorry; I would have edited the post but the site does not allow me. The error message I posted was from when I had my custom debugging helpers on. Them being on or not (or even present) has no impact on this failure, but they do change the error message slightly and I didn't want to confuse the issue with that oversight. Here is a version of the error with no involvement from those:
<718python theDumper.fetchVariables({"allowinferiorcalls":1,"autoderef":1,"context":"","displaystring
>&"python theDumper.fetchVariables({\"allowinferiorcalls\":1,\"autoderef\":1,\"context\":\"\",\"displaystringlimit\":300,\"dyntype\":1,\"expanded\":{\"inspect\":100,\"local\":100,\"local.boxHigh\":100,\"return\":100,\"watch\":100},\"fancy\":0,\"formats\":{},\"nativemixed\":0,\"partialvar\":\"local.boxHigh\",\"passexceptions\":0,\"qobjectnames\":1,\"qtnamespace\":\"\",\"qtversion\":331539,\"resultvarname\":\"\",\"stringcutoff\":10000,\"timestamps\":0,\"token\":718,\"typeformats\":{},\"watchers\":[{\"exp\":\"6d446972656374696f6e\",\"iname\":\"watch.0\"}]})\n"
>~"bridgemessage={msg=\"b' COULD NOT ACCESS FIELD: union {...}'\"},\n"
>~"bridgemessage={msg=\"b' COULD NOT ACCESS FIELD: union {...}'\"},\n"
>~"bridgemessage={msg=\"b' COULD NOT ACCESS FIELD: union {...}'\"},\n"
>~"result={token=\"718\",data=[{iname=\"local.boxHigh\",name=\"boxHigh\",address=\"0x555555f48b78\",size=\"12\",numchild=\"1\",sortable=\"1\",children=[],type=\"glm::vec3&\",value=\"\",},{iname=\"watch.0\",wname=\"6D446972656374696F6E\",numchild=\"0\",type=\" \",value=\"<no such value>\",},],typeinfo=[],partial=\"1\",runtime=\"0.012951968994457275\",counts={}}\n"
>718^done
<Rebuild Watchmodel 92 @ 23:19:10.586 [3141ms] >
[GDB] Finished retrieving data.