BC: Fix BC undo command.

src/Model/BoundaryCondition/BoundaryCondition.py

@@ -582,10 +582,10 @@ class BoundaryCondition(SQLSubModel):
         self._data.index(bc)
 
     def get_i(self, index):
-        if len(self.data) == 0:
+        if len(self._data) == 0:
             return None
 
-        return self.data[index]
+        return self._data[index]
 
     def get_range(self, _range):
         lst = []