fix: don't try to access contacts if they are none

Author: lachlangrose

LoopStructural/modelling/input/process_data.py

@@ -486,6 +486,8 @@ def _update_feature_names(self, dataframe):
 
     @property
     def contacts(self):
+        if self._contacts is None:
+            return None
         contacts = self._contacts.copy()
         if self._use_thickness:
             contacts["val"] = np.nan