SlicerRT allows calling CalculateDose without specific target that makes pyradplan crash

[ferdymercury]

Bug Description

In SlicerRT, one can use a isocenter defined by hand, and you do not need to specify a target contour / volume. Then if you calculateDose, pyradplan crashes with:

ValueError: need at least one array to concatenate
"/opt/Slicer-5.13.0-2026-06-30-linux-amd64/slicer.org/Extensions-34833/SlicerRT/lib/Slicer-5.13/qt-scripted-modules/DoseEngines/pyRadPlanEngine.py" : clone: Failed to call mandatory calculateDoseUsingEngine method! If it is implemented, please see python output for errors.
"/opt/Slicer-5.13.0-2026-06-30-linux-amd64/slicer.org/Extensions-34833/SlicerRT/lib/Slicer-5.13/qt-scripted-modules/DoseEngines/pyRadPlanEngine.py" : Calling calculateDoseUsingEngine from Python dose engine
Traceback (most recent call last):
  File "/opt/Slicer-5.13.0-2026-06-30-linux-amd64/slicer.org/Extensions-34833/SlicerRT/lib/Slicer-5.13/qt-scripted-modules/DoseEngines/pyRadPlanEngine.py", line 80, in calculateDoseUsingEngine
    stf = generate_stf(ct, cst, pln)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/Slicer-5.13.0-2026-06-30-linux-amd64/lib/Python/lib/python3.12/site-packages/pyRadPlan/stf/_generate_stf.py", line 24, in generate_stf
    return validate_stf(stfgen.generate(ct, cst))
                        ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/Slicer-5.13.0-2026-06-30-linux-amd64/lib/Python/lib/python3.12/site-packages/pyRadPlan/stf/generators/_base.py", line 82, in generate
    self._initialize_patient_geometry()
  File "/opt/Slicer-5.13.0-2026-06-30-linux-amd64/lib/Python/lib/python3.12/site-packages/pyRadPlan/stf/generators/_base.py", line 123, in _initialize_patient_geometry
    self._target_mask = self._cst.target_union_mask()
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/Slicer-5.13.0-2026-06-30-linux-amd64/lib/Python/lib/python3.12/site-packages/pyRadPlan/cst/_cst.py", line 175, in target_union_mask
    target_indices = self.target_union_voxels(order="numpy")
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/Slicer-5.13.0-2026-06-30-linux-amd64/lib/Python/lib/python3.12/site-packages/pyRadPlan/cst/_cst.py", line 170, in target_union_voxels
    return np.unique(np.concatenate(target_indices))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Steps to Reproduce

• Import Protons_01.zip into SlicerRT
• go to external beams module,
• click on Ion Plan
• then select active RT plan,
• use CT grid as dose output new volume
• click on Calculate Dose --> no output is given.

Expected Behavior

Ideally, pyradplan should just print a warning that target_mask is empty and continue the calculation since it shouldn't be mandatory.

Actual Behavior

No output dose, silent crash of pyradplan.

Environment

Slicer 5.13.0 nightly from Jul-30-2026

Additional Context

Thanks in advance!