[isabelle] ML stack trace?

i would like to find out more information when the isabelle gives "empty
result sequence".
it seems this exception is raised by a line proof_history.ML, so i tried to
use polyml
debugger to set a breakpoint, recompile the source adding
Poly.Compiler.debug := true
then in proof general use "ML {* open PolyML.Debug; breakAt
(proof_history.ML 70); *}"
but it doesnt seem to work. is there a way to get the stack trace?

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