ForceFieldHelpers: fix “No Python class registered” error (#8800)

Fixes github issue #8597.

* UFFGetMoleculeForceField was returning
PyForceField* before the class was registered via rdForceField,
which caused “No Python class registered” errors in Python when
using multiple conformers.

* This patch forces rdForceField to be
imported during rdForceFieldHelpers module init, ensuring the
class is always registered before helper functions are used.

* Add unit test

Code/GraphMol/ForceFieldHelpers/Wrap/rdForceFields.cpp

@@ -248,6 +248,8 @@ PyObject *getUFFVdWParams(const RDKit::ROMol &mol, const unsigned int idx1,
 }  // namespace ForceFields
 
 BOOST_PYTHON_MODULE(rdForceFieldHelpers) {
+   boost::python::import("rdkit.ForceField.rdForceField");
+
   python::scope().attr("__doc__") =
       "Module containing functions to handle force fields";
 

Code/GraphMol/ForceFieldHelpers/Wrap/testHelpers.py

@@ -5,7 +5,7 @@ import numpy
 
 from rdkit import Chem, RDConfig
 from rdkit.Chem import ChemicalForceFields, rdDistGeom
-
+from rdkit.Chem.rdForceFieldHelpers import UFFGetMoleculeForceField
 
 def feq(v1, v2, tol2=1e-4):
   return abs(v1 - v2) <= tol2
@@ -408,5 +408,14 @@ M  END"""
     self.assertAlmostEqual((posa - posb).Length(), 100, delta=10e-5)
 
 
+  def test_uff_get_forcefield_runs(self):
+      mol = Chem.MolFromSmiles("CCO")
+      mol = Chem.AddHs(mol)
+      rdDistGeom.EmbedMolecule(mol, randomSeed=42)
+      ff = UFFGetMoleculeForceField(mol, ignoreInterfragInteractions=False)
+      self.assertIsNotNone(ff)
+      self.assertTrue(hasattr(ff, "CalcEnergy"))
+
+
 if __name__ == "__main__":
   unittest.main()