diff --git a/src/aiida_workgraph/config.py b/src/aiida_workgraph/config.py
index b97fb8cd..519747ec 100644
--- a/src/aiida_workgraph/config.py
+++ b/src/aiida_workgraph/config.py
@@ -1,6 +1,8 @@
 import json
 from aiida.manage.configuration.settings import AIIDA_CONFIG_FOLDER
 
+WORKGRAPH_EXTRA_KEY = "_workgraph"
+
 
 def load_config() -> dict:
     """Load the configuration from the config file."""
diff --git a/src/aiida_workgraph/engine/workgraph.py b/src/aiida_workgraph/engine/workgraph.py
index 5b79c27a..6cfe0635 100644
--- a/src/aiida_workgraph/engine/workgraph.py
+++ b/src/aiida_workgraph/engine/workgraph.py
@@ -308,8 +308,9 @@ def setup_ctx_workgraph(self, wgdata: t.Dict[str, t.Any]) -> None:
     def read_wgdata_from_base(self) -> t.Dict[str, t.Any]:
         """Read workgraph data from base.extras."""
         from aiida_workgraph.orm.function_data import PickledLocalFunction
+        from aiida_workgraph.config import WORKGRAPH_EXTRA_KEY
 
-        wgdata = self.node.base.extras.get("_workgraph")
+        wgdata = self.node.base.extras.get(WORKGRAPH_EXTRA_KEY)
         for name, task in wgdata["tasks"].items():
             wgdata["tasks"][name] = deserialize_unsafe(task)
             for _, input in wgdata["tasks"][name]["inputs"].items():
diff --git a/src/aiida_workgraph/utils/__init__.py b/src/aiida_workgraph/utils/__init__.py
index 55d2fb2e..9b6e91d6 100644
--- a/src/aiida_workgraph/utils/__init__.py
+++ b/src/aiida_workgraph/utils/__init__.py
@@ -322,10 +322,11 @@ def get_workgraph_data(process: Union[int, orm.Node]) -> Optional[Dict[str, Any]
     """Get the workgraph data from the process node."""
     from aiida.orm.utils.serialize import deserialize_unsafe
     from aiida.orm import load_node
+    from aiida_workgraph.config import WORKGRAPH_EXTRA_KEY
 
     if isinstance(process, int):
         process = load_node(process)
-    wgdata = process.base.extras.get("_workgraph", None)
+    wgdata = process.base.extras.get(WORKGRAPH_EXTRA_KEY, None)
     if wgdata is None:
         return
     for name, task in wgdata["tasks"].items():
diff --git a/src/aiida_workgraph/utils/analysis.py b/src/aiida_workgraph/utils/analysis.py
index 477a2f27..f9d43716 100644
--- a/src/aiida_workgraph/utils/analysis.py
+++ b/src/aiida_workgraph/utils/analysis.py
@@ -3,6 +3,7 @@
 # import datetime
 from aiida.orm import ProcessNode
 from aiida.orm.utils.serialize import serialize, deserialize_unsafe
+from aiida_workgraph.config import WORKGRAPH_EXTRA_KEY
 
 
 class WorkGraphSaver:
@@ -223,7 +224,7 @@ def insert_workgraph_to_db(self) -> None:
         # nodes is a copy of tasks, so we need to pop it out
         self.wgdata["error_handlers"] = serialize(self.wgdata["error_handlers"])
         self.wgdata["context"] = serialize(self.wgdata["context"])
-        self.process.base.extras.set("_workgraph", self.wgdata)
+        self.process.base.extras.set(WORKGRAPH_EXTRA_KEY, self.wgdata)
 
     def save_task_states(self) -> Dict:
         """Get task states."""
@@ -277,7 +278,7 @@ def get_wgdata_from_db(
     ) -> Optional[Dict]:
 
         process = self.process if process is None else process
-        wgdata = process.base.extras.get("_workgraph", None)
+        wgdata = process.base.extras.get(WORKGRAPH_EXTRA_KEY, None)
         if wgdata is None:
             print("No workgraph data found in the process node.")
             return
@@ -318,7 +319,7 @@ def exist_in_db(self) -> bool:
         Returns:
             bool: _description_
         """
-        if self.process.base.extras.get("_workgraph", None) is not None:
+        if self.process.base.extras.get(WORKGRAPH_EXTRA_KEY, None) is not None:
             return True
         return False
 
diff --git a/src/aiida_workgraph/workgraph.py b/src/aiida_workgraph/workgraph.py
index aad7b10e..dfd371bf 100644
--- a/src/aiida_workgraph/workgraph.py
+++ b/src/aiida_workgraph/workgraph.py
@@ -362,8 +362,7 @@ def load(cls, pk: int) -> Optional["WorkGraph"]:
         process = aiida.orm.load_node(pk)
         wgdata = get_workgraph_data(process)
         if wgdata is None:
-            print("No workgraph data found in the process node.")
-            return
+            raise ValueError(f"WorkGraph data not found for process {pk}.")
         wg = cls.from_dict(wgdata)
         wg.process = process
         wg.update()
diff --git a/tests/test_error_handler.py b/tests/test_error_handler.py
index a7e7d89d..d1eff81f 100644
--- a/tests/test_error_handler.py
+++ b/tests/test_error_handler.py
@@ -38,6 +38,7 @@ def handle_negative_sum(task: Task):
             }
         },
     )
+    assert len(wg.error_handlers) == 1
     wg.submit(
         inputs={
             "add1": {"code": add_code, "x": orm.Int(1), "y": orm.Int(-2)},
diff --git a/tests/test_workgraph.py b/tests/test_workgraph.py
index 1fa1366b..f65a7329 100644
--- a/tests/test_workgraph.py
+++ b/tests/test_workgraph.py
@@ -28,8 +28,29 @@ def test_add_task():
     assert len(wg.links) == 1
 
 
+def test_show_state(wg_calcfunction):
+    from io import StringIO
+    import sys
+
+    # Redirect stdout to capture prints
+    captured_output = StringIO()
+    sys.stdout = captured_output
+    # Call the method
+    wg_calcfunction.name = "test_show_state"
+    wg_calcfunction.show()
+    # Reset stdout
+    sys.stdout = sys.__stdout__
+    # Check the output
+    output = captured_output.getvalue()
+    assert "WorkGraph: test_show_state, PK: None, State: CREATED" in output
+    assert "sumdiff1" in output
+    assert "PLANNED" in output
+
+
 def test_save_load(wg_calcfunction):
     """Save the workgraph"""
+    from aiida_workgraph.config import WORKGRAPH_EXTRA_KEY
+
     wg = wg_calcfunction
     wg.name = "test_save_load"
     wg.save()
@@ -38,6 +59,12 @@ def test_save_load(wg_calcfunction):
     assert wg.process.label == "test_save_load"
     wg2 = WorkGraph.load(wg.process.pk)
     assert len(wg.tasks) == len(wg2.tasks)
+    # remove the extra
+    wg.process.base.extras.delete(WORKGRAPH_EXTRA_KEY)
+    with pytest.raises(
+        ValueError, match=f"WorkGraph data not found for process {wg.process.pk}."
+    ):
+        WorkGraph.load(wg.process.pk)
 
 
 def test_organize_nested_inputs():
@@ -86,7 +113,7 @@ def test_reset_message(wg_calcjob):
     assert "Action: reset. {'add2'}" in report
 
 
-def test_restart(wg_calcfunction):
+def test_restart_and_reset(wg_calcfunction):
     """Restart from a finished workgraph.
     Load the workgraph, modify the task, and restart the workgraph.
     Only the modified node and its child tasks will be rerun."""
@@ -109,6 +136,10 @@ def test_restart(wg_calcfunction):
     assert wg1.tasks["sumdiff2"].node.pk != wg.tasks["sumdiff2"].pk
     assert wg1.tasks["sumdiff3"].node.pk != wg.tasks["sumdiff3"].pk
     assert wg1.tasks["sumdiff3"].node.outputs.sum == 19
+    wg1.reset()
+    assert wg1.process is None
+    assert wg1.tasks["sumdiff3"].process is None
+    assert wg1.tasks["sumdiff3"].state == "PLANNED"
 
 
 def test_extend_workgraph(decorated_add_multiply_group):
diff --git a/tests/widget/test_widget.py b/tests/widget/test_widget.py
index 3c934a5b..33c2ad7e 100644
--- a/tests/widget/test_widget.py
+++ b/tests/widget/test_widget.py
@@ -14,6 +14,8 @@ def test_workgraph_widget(wg_calcfunction):
     # to_html
     data = wg.to_html()
     assert isinstance(data, IFrame)
+    # check _repr_mimebundle_ is working
+    data = wg._repr_mimebundle_()
 
 
 def test_workgraph_task(wg_calcfunction):
@@ -26,3 +28,5 @@ def test_workgraph_task(wg_calcfunction):
     # to html
     data = wg.tasks["sumdiff2"].to_html()
     assert isinstance(data, IFrame)
+    # check _repr_mimebundle_ is working
+    data = wg._repr_mimebundle_()