fix: guard interpolator and grid search against edge cases

autofit/interpolator/abstract.py

@@ -95,6 +95,11 @@ def __getitem__(self, item: Equality) -> ModelInstance:
             value_map = self._value_map(item.path)
             x = sorted(value_map)
 
+            if not self.instances:
+                raise IndexError(
+                    "Cannot interpolate: no instances have been added to the interpolator."
+                )
+
             instance = self.instances[0]
             new_instance = copy.copy(instance)
 

autofit/non_linear/grid/grid_search/result.py

@@ -303,7 +303,12 @@ def log_evidences(self, relative_to_value: float = 0.0) -> GridList:
             The value to subtract from every log likelihood, for example if Bayesian model comparison is performed
             on the grid search and the subtracted value is the maximum log likelihood of a previous search.
         """
-        return [sample.log_evidence - relative_to_value for sample in self.samples]
+        return [
+            sample.log_evidence - relative_to_value
+            if sample.log_evidence is not None
+            else None
+            for sample in self.samples
+        ]
 
     def figure_of_merits(
         self, use_log_evidences: bool, relative_to_value: float = 0.0