product/Workflow&erp5_workflow_test: fix can't access workflow state after change State Variable

we have such error:
....
if context.getValidationState() in ["draft", "deleted", "cancelled", "archived"]:
  Module Products.ERP5Type.Accessor.WorkflowState, line 60, in __call__
    return wf._getWorkflowStateOf(instance, id_only=1)
  Module Products.ERP5Type.Core.Workflow, line 609, in _getWorkflowStateOf
    state_id = 'state_' + status.get(self.getStateVariable(), None)
TypeError: cannot concatenate 'str' and 'NoneType' objects

bt5/erp5_workflow_test/TestTemplateItem/portal_components/test.erp5.testWorkflowAndDCWorkflow.py

@@ -14,6 +14,7 @@ class TestERP5WorkflowMixin(TestWorkflowMixin):
     module = self.portal.workflow_test_module
     module.manage_delObjects(list(module.objectIds()))
 
+    self.portal.portal_workflow[self.initial_dc_workflow_id].setStateVariable('validation_state')
     self.copyWorkflow(self.portal.portal_workflow, self.initial_dc_workflow_id, self.workflow_id)
     self.copyWorkflow(self.portal.portal_workflow, self.initial_dc_interaction_workflow_id, self.interaction_workflow_id)
 
@@ -191,6 +192,16 @@ class TestERP5WorkflowMixin(TestWorkflowMixin):
     self.checkDocumentState(new_object, 'validated')
     self.assertEqual(workflow_tool.isTransitionPossible(new_object, 'invalidate'), 1)
 
+  def test_13_testAccessWorkflowStateAfterChangeStateVariable(self):
+    new_object = self.getTestObject()
+    self.tic()
+    self.assertEqual(new_object.getValidationState(), 'draft')
+    self.portal.portal_workflow[self.initial_dc_workflow_id].setStateVariable('simulation_state')
+    self.portal.portal_workflow[self.workflow_id].setStateVariable('simulation_state')
+    self.tic()
+    self.assertEqual(new_object.getSimulationState(), 'draft')
+    self.tic()
+
 class TestConvertedWorkflow(TestERP5WorkflowMixin):
   """
     Tests Converted Workflow which generated dynamically from DCWorkflow.

product/ERP5Type/Core/Workflow.py

@@ -606,9 +606,11 @@ class Workflow(XMLObject):
     if status is None:
       state = self.getSourceValue()
     else:
-      state_id = 'state_' + status.get(self.getStateVariable(), None)
+      state_parameter = status.get(self.getStateVariable(), None)
+      if state_parameter:
+        state_id = 'state_' + state_parameter
         state = self._getOb(state_id)
-      if state is None:
+      if (state_parameter is None) or (state is None):
         state = self.getSourceValue()
     if id_only:
       if state is None: