fix: lazy initialization of the checkpointer

Author: zszia

solnlib/modular_input/modular_input.py

@@ -174,7 +174,7 @@ def checkpointer(self) -> checkpointer.Checkpointer:
         """
 
         if self._checkpointer is not None:
-            return self._checkpoint_dir
+            return self._checkpointer
 
         self._checkpointer = self._create_checkpointer()
         return self._checkpointer