BHS: Fix undo commands.

src/Model/HydraulicStructures/HydraulicStructures.py

@@ -425,6 +425,9 @@ class HydraulicStructure(SQLSubModel):
         return self.lst.copy()
 
     def basic_structure(self, index: int):
+        if len(self._data) == 0:
+            return None
+
         return self.lst[index]
 
     def add(self, index: int):