Arbor crashes on GPU if a mechanism is probed that is not placed

Arbor crashes with IndexError: _Map_base::at on GPU if a mechanism is probed that is not placed. This is different from the behaviour on CPU where Arbor runs and, as expected, samples are empty when retrieved after the simulation. The attached script reproduces the bug on 27db9a7f700bb05680a811ba3445e734d61aacf6.

The line that triggers is:

arbor/arbor/fvm_lowered_cell_impl.hpp

Line 1062 in b63cca1

auto& placed_instances = R.cell.synapses().at(p.mechanism);

script:

single_cell_recipe.py.gz

stack trace:

gdb.txt.gz

A suggestion: I think it would be nice to throw a warning any time a mechanism is probed which has not been placed. That may save some development time for CPU setups as well.