diff --git a/.gitignore b/.gitignore
index 972b1a9..7af088b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,6 +20,7 @@
 /MANIFEST
 /crm.sqlite3
 db*.sqlite3
+dj*.sqlite3
 db*.sqlite3-journal
 node_modules/
 /docs/_build/**
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 86751c8..0af6c2f 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -2,6 +2,15 @@
 Changelog
 =========
 
+2.2.8 GIT VERSION
+-----------------
+
+- No exception raiased by Process/Task models in case if flow class deleted, but
+  still referenced in DB
+- Fix jsonstore.DecimalField serialization
+- Add missing 'index' view for celery.Task node
+- Allow to revive flow.Subprocess and flow.NSubprocess nodes from error state
+
 2.2.7 2024-08-16
 ----------------
 
@@ -412,7 +421,7 @@ This is the cumulative release with many backward incompatibility changes.
 
 * Activation now passed as a request attribute. You need to remove
   explicit activation parameter from view function signature, and use
-  request.activation instead. 
+  request.activation instead.
 
 * Built-in class based views are renamed, to be more consistent. Check
   the documentation to find a new view name.
@@ -429,7 +438,7 @@ This is the cumulative release with many backward incompatibility changes.
   and signal handlers reusable over different flows.
 
 * Flow namespace are no longer hard-coded. Flow views now can be
-  attached to any namespace in a URL config. 
+  attached to any namespace in a URL config.
 
 * flow_start_func, flow_start_signal decorators need to be used for
   the start nodes handlers. Decorators would establish a proper
diff --git a/tox.ini b/tox.ini
index 50d6b45..b29d37e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -68,6 +68,11 @@ deps =
   tblib==3.0.0
   twine==4.0.2
 
+  # typing
+  django-stubs==5.0.4
+  django-filter-stubs==0.1.3
+  mypy==1.11.1
+
   # packaging
   pyc-wheel==1.2.7
   setuptools
diff --git a/viewflow/fsm/typing.py b/viewflow/fsm/typing.py
index 1a2fd9a..bdc1d8b 100644
--- a/viewflow/fsm/typing.py
+++ b/viewflow/fsm/typing.py
@@ -13,7 +13,7 @@ if TYPE_CHECKING:
 
 UserModel = Any
 StateValue = Any
-Condition = Union[ThisObject, Callable[[object, object], bool]]
+Condition = Union[ThisObject, Callable[[object], bool]]
 Permission = Union[ThisObject, Callable[[object, Any], bool]]
 StateTransitions = Mapping["TransitionMethod", List["Transition"]]
 TransitionFunction = Callable[..., Any]
diff --git a/viewflow/jsonstore.py b/viewflow/jsonstore.py
index 52da443..a7e76cf 100644
--- a/viewflow/jsonstore.py
+++ b/viewflow/jsonstore.py
@@ -8,6 +8,7 @@
 import copy
 import json
 from datetime import date, datetime
+from decimal import Decimal
 from functools import partialmethod
 
 from django.core.exceptions import FieldError
@@ -190,7 +191,13 @@ class DateTimeField(JSONFieldMixin, fields.DateTimeField):
 
 
 class DecimalField(JSONFieldMixin, fields.DecimalField):
-    pass
+    def to_json(self, value):
+        if value is not None:
+            return str(value)
+
+    def from_json(self, value):
+        if value is not None:
+            return Decimal(value)
 
 
 class EmailField(JSONFieldMixin, fields.EmailField):
diff --git a/viewflow/urls/base.py b/viewflow/urls/base.py
index 7290a93..2662816 100644
--- a/viewflow/urls/base.py
+++ b/viewflow/urls/base.py
@@ -8,10 +8,9 @@ import copy
 import types
 import warnings
 from collections import namedtuple, OrderedDict
-from typing import Optional, Dict, Any, List, Union
-
+from typing import Optional, Dict, Any, List
 from django.views.generic import RedirectView
-from django.urls import URLPattern, URLResolver, include, path, reverse
+from django.urls import ResolverMatch, URLPattern, URLResolver, include, path, reverse
 from django.urls.resolvers import RoutePattern
 
 from viewflow.utils import (
@@ -40,7 +39,7 @@ class _URLResolver(URLResolver):
         self.extra = kwargs.pop("extra", {})
         super(_URLResolver, self).__init__(*args, **kwargs)
 
-    def resolve(self, *args, **kwargs) -> str:
+    def resolve(self, *args, **kwargs) -> ResolverMatch:
         result = super(_URLResolver, self).resolve(*args, **kwargs)
         if not isinstance(result.url_name, _UrlName):
             result.url_name = _UrlName(result.url_name)
diff --git a/viewflow/urls/sites.py b/viewflow/urls/sites.py
index 3b30e9a..0131ee3 100644
--- a/viewflow/urls/sites.py
+++ b/viewflow/urls/sites.py
@@ -17,7 +17,7 @@ class AppMenuMixin:
     """A route that can be listed in an Application menu."""
 
     title = None
-    icon = Icon("view_carousel")
+    icon: Icon | str = Icon("view_carousel")
 
     def __init__(self, **kwargs):
         super().__init__(**kwargs)
@@ -47,7 +47,7 @@ class AppMenuMixin:
 
 class Application(IndexViewMixin, Viewset):
     title = ""
-    icon = Icon("view_module")
+    icon: Icon | str = Icon("view_module")
     menu_template_name = "viewflow/includes/app_menu.html"
     base_template_name = "viewflow/base_page.html"
     permission = None
diff --git a/viewflow/utils.py b/viewflow/utils.py
index 0407562..56caf61 100644
--- a/viewflow/utils.py
+++ b/viewflow/utils.py
@@ -230,6 +230,7 @@ def get_object_data(obj: models.Model) -> Iterator[Tuple[models.Field, str, Any]
 
     if (
         hasattr(obj, "artifact_object_id")
+        and hasattr(obj, "artifact")
         and obj.artifact_object_id
         and obj.artifact is not None
     ):
diff --git a/viewflow/views/base.py b/viewflow/views/base.py
index 7d6c388..16b97ab 100644
--- a/viewflow/views/base.py
+++ b/viewflow/views/base.py
@@ -5,6 +5,7 @@
 # LICENSE_EXCEPTION and the Commercial licence defined in file 'COMM_LICENSE',
 # which is part of this source code package.
 
+from typing import Any, List, Union
 from django import forms
 from viewflow.utils import viewprop
 from viewflow.forms import Span
@@ -29,7 +30,7 @@ class FormLayoutMixin(object):
     Mixin for FormView to infer View.fields definition from form Layout.
     """
 
-    form_class = None
+    form_class: Any = None
 
     @viewprop
     def layout(self):
@@ -37,7 +38,7 @@ class FormLayoutMixin(object):
             return self.form_class.layout
 
     @viewprop
-    def fields(self):
+    def fields(self) -> Any:
         if self.form_class is None:
             if self.layout is not None:
                 return _collect_elements(self.layout)
diff --git a/viewflow/workflow/fields.py b/viewflow/workflow/fields.py
index 2ba8908..b41a572 100644
--- a/viewflow/workflow/fields.py
+++ b/viewflow/workflow/fields.py
@@ -94,6 +94,8 @@ class FlowReferenceField(models.CharField):
             return import_flow_by_ref(value)
         except LookupError:
             return None
+        except ImportError:
+            return None
 
     def get_prep_value(self, value):  # noqa D1o2
         if value and not isinstance(value, str):
@@ -118,7 +120,11 @@ class TaskReferenceField(models.CharField):
     def from_db_value(self, value, expression, connection):
         if value is None:
             return value
-        return import_task_by_ref(value)
+
+        try:
+            return import_task_by_ref(value)
+        except ImportError:
+            return None
 
     def get_prep_value(self, value):  # noqa D102
         if value and not isinstance(value, str):
diff --git a/viewflow/workflow/flow/mixins.py b/viewflow/workflow/flow/mixins.py
index bbcfa8c..87d4cd2 100644
--- a/viewflow/workflow/flow/mixins.py
+++ b/viewflow/workflow/flow/mixins.py
@@ -1,3 +1,5 @@
+from typing import Type, Optional
+from django.views import View
 from django.urls import path
 from viewflow import viewprop
 from viewflow.urls import ViewsetMeta
@@ -7,7 +9,7 @@ from . import utils
 class NodeDetailMixin(metaclass=ViewsetMeta):
     """Task detail view."""
 
-    index_view_class = None
+    index_view_class: Optional[Type[View]] = None
 
     @viewprop
     def index_view(self):
@@ -24,7 +26,7 @@ class NodeDetailMixin(metaclass=ViewsetMeta):
                 name="index",
             )
 
-    detail_view_class = None
+    detail_view_class: Optional[Type[View]] = None
 
     @viewprop
     def detail_view(self):
@@ -49,7 +51,7 @@ class NodeDetailMixin(metaclass=ViewsetMeta):
 class NodeExecuteMixin(metaclass=ViewsetMeta):
     """Re-execute a gate manually."""
 
-    execute_view_class = None
+    execute_view_class: Optional[Type[View]] = None
 
     @viewprop
     def execute_view(self):
@@ -70,7 +72,7 @@ class NodeExecuteMixin(metaclass=ViewsetMeta):
 class NodeUndoMixin(metaclass=ViewsetMeta):
     """Allow to undo a completed task."""
 
-    undo_view_class = None
+    undo_view_class: Optional[Type[View]] = None
 
     @viewprop
     def undo_view(self):
@@ -94,7 +96,7 @@ class NodeUndoMixin(metaclass=ViewsetMeta):
 class NodeCancelMixin(metaclass=ViewsetMeta):
     """Cancel a task action."""
 
-    cancel_view_class = None
+    cancel_view_class: Optional[Type[View]] = None
 
     @viewprop
     def cancel_view(self):
@@ -120,7 +122,7 @@ class NodeCancelMixin(metaclass=ViewsetMeta):
 class NodeReviveMixin(metaclass=ViewsetMeta):
     """Review a canceled task"""
 
-    revive_view_class = None
+    revive_view_class: Optional[Type[View]] = None
 
     @viewprop
     def revive_view(self):
diff --git a/viewflow/workflow/flow/nodes.py b/viewflow/workflow/flow/nodes.py
index 3c73acc..23014fd 100644
--- a/viewflow/workflow/flow/nodes.py
+++ b/viewflow/workflow/flow/nodes.py
@@ -420,6 +420,7 @@ try:
         mixins.NodeDetailMixin,
         mixins.NodeCancelMixin,
         mixins.NodeUndoMixin,
+        mixins.NodeReviveMixin,
         nodes.Subprocess,
     ):
         """
@@ -452,11 +453,13 @@ try:
         detail_view_class = views.DetailTaskView
         cancel_view_class = views.CancelTaskView
         undo_view_class = views.UndoTaskView
+        revive_view_class = views.ReviveTaskView
 
     class NSubprocess(
         mixins.NodeDetailMixin,
         mixins.NodeCancelMixin,
         mixins.NodeUndoMixin,
+        mixins.NodeReviveMixin,
         nodes.NSubprocess,
     ):
         """
@@ -491,6 +494,7 @@ try:
         detail_view_class = views.DetailTaskView
         cancel_view_class = views.CancelTaskView
         undo_view_class = views.UndoTaskView
+        revive_view_class = views.ReviveTaskView
 
 except AttributeError:
     """Pro-only functionality"""
@@ -504,6 +508,22 @@ class Switch(
     mixins.NodeReviveMixin,
     nodes.Switch,
 ):
+    """
+    Gateway that selects one of the outgoing node.
+
+    Activates first node with matched condition.
+
+    Example::
+
+        select_responsible_person = (
+            flow.Switch()
+            .Case(this.dean_approval, lambda act: a.process.need_dean)
+            .Case(this.head_approval, lambda act: a.process.need_head)
+            .Default(this.supervisor_approval)
+        )
+
+    """
+
     index_view_class = views.IndexTaskView
     detail_view_class = views.DetailTaskView
     cancel_view_class = views.CancelTaskView
diff --git a/viewflow/workflow/models.py b/viewflow/workflow/models.py
index b489bf7..591390a 100644
--- a/viewflow/workflow/models.py
+++ b/viewflow/workflow/models.py
@@ -43,6 +43,9 @@ class AbstractProcess(models.Model):
     @property
     def brief(self):
         """Quick textual process state representation for end user."""
+        if self.flow_class is None:
+            return None
+
         template_content = ""
 
         if self.finished:
@@ -175,6 +178,9 @@ class AbstractTask(models.Model):
 
     @property
     def title(self):
+        if self.flow_task is None:
+            return None
+
         if self.flow_task.task_title:
             return self.flow_task.task_title
         return _(str(self.flow_task))