Tree build crashes if no sources present

[inducer]

  return DirectionalSourceDerivative(
/home/kirby/Code/firedrake-complex/src/sumpy/sumpy/kernel.py:1316: UserWarning: specified the name of the direction vector
  return type(kernel)(
/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/refinement.py:476: RefinerNotConvergedWarning: QBX layer potential source refiner did not terminate after 0 iterations (the maximum). You may call 'refine_geometry_collection()' manually and pass 'visualize=True' to see what area of the geometry is causing trouble. If the issue is disturbance of expansion disks, you may pass a slightly increased value (currently: 0.025) for 'expansion_disturbance_tolerance'. As a last resort, you may use Python's warning filtering mechanism to not treat this warning as an error. The criteria triggering refinement in each iteration were: . 
  warn(
/home/kirby/Code/firedrake-complex/lib/python3.8/site-packages/numpy/core/fromnumeric.py:86: VisibleDeprecationWarning: Creating an ndarray from nested sequences exceeding the maximum number of dimensions of 32 is deprecated. If you mean to do this, you must specify 'dtype=object' when creating the ndarray.
  return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/refinement.py:476: RefinerNotConvergedWarning: QBX layer potential source refiner did not terminate after 0 iterations (the maximum). You may call 'refine_geometry_collection()' manually and pass 'visualize=True' to see what area of the geometry is causing trouble. If the issue is disturbance of expansion disks, you may pass a slightly increased value (currently: 0.025) for 'expansion_disturbance_tolerance'. As a last resort, you may use Python's warning filtering mechanism to not treat this warning as an error. The criteria triggering refinement in each iteration were: . 
  warn(
/home/kirby/Code/firedrake-complex/src/loopy/loopy/target/execution.py:193: ParameterFinderWarning: Unable to generate code to automatically find 'nunit_dofs' from the shape of 'result':
division with remainder in linear solve for 'nunit_dofs'
  warn("Unable to generate code to automatically "
/home/kirby/Code/firedrake-complex/src/boxtree/boxtree/tree_build.py:380: RuntimeWarning: overflow encountered in subtract
  bbox["max_"+ax] - bbox["min_"+ax]
Traceback (most recent call last):
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 822, in _get_qbx_discretization
    discr = self._get_discr_from_cache(geometry, discr_stage)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 782, in _get_discr_from_cache
    raise KeyError(
KeyError: "cached discretization does not exist on '{geometry}'for stage '{discr_stage}'"

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/refinement.py", line 893, in _refine_for_global_qbx
    discr, conn = get_from_cache(*ds)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/refinement.py", line 863, in get_from_cache
    discr = places._get_discr_from_cache(geometry, to_ds)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 782, in _get_discr_from_cache
    raise KeyError(
KeyError: "cached discretization does not exist on '{geometry}'for stage '{discr_stage}'"

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "mi_decoupled_nl.py", line 241, in <module>
    Bctx.dlp.get_pot_and_grad(x, complex(kt))
  File "/home/kirby/Documents/mibc/code/laypot.py", line 97, in get_pot_and_grad
    op_grad_x = self.bound_op_and_grad(self.actx, u=density, k=kappa)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 1157, in __call__
    return self.eval(kwargs, array_context=array_context)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 1131, in eval
    return self.code.execute(exec_mapper)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/compiler.py", line 404, in execute
    self.get_exec_function(insn, exec_mapper)(
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 313, in exec_assign
    return [(name, evaluate(expr))
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 313, in <listcomp>
    return [(name, evaluate(expr))
  File "/home/kirby/Code/firedrake-complex/lib/python3.8/site-packages/pymbolic/mapper/__init__.py", line 129, in __call__
    return method(expr, *args, **kwargs)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 279, in map_interpolation
    conn = self.places.get_connection(expr.from_dd, expr.to_dd)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 853, in get_connection
    return connection_from_dds(self, from_dd, to_dd)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/dof_connection.py", line 234, in connection_from_dds
    to_discr = places.get_discretization(to_dd.geometry, to_dd.discr_stage)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 881, in get_discretization
    return self._get_qbx_discretization(geometry, discr_stage)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/symbolic/execution.py", line 829, in _get_qbx_discretization
    _refine_for_global_qbx(self, dofdesc,
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/refinement.py", line 895, in _refine_for_global_qbx
    discr, conn = _refine_qbx_stage2(
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/refinement.py", line 736, in _refine_qbx_stage2
    tree = wrangler.build_tree(places,
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/utils.py", line 129, in build_tree
    return build_tree_with_qbx_metadata(
  File "/home/kirby/Code/firedrake-complex/lib/python3.8/site-packages/pytools/__init__.py", line 2511, in wrapper
    return wrapped(*args, **kwargs)
  File "/home/kirby/Code/firedrake-complex/src/pytential/pytential/qbx/utils.py", line 321, in build_tree_with_qbx_metadata
    tree, evt = tree_builder(queue, particles,
  File "/home/kirby/Code/firedrake-complex/src/boxtree/boxtree/tree_build.py", line 461, in __call__
    assert nboxes_guess > 0
AssertionError```