Traceback (most recent call last):
  File "/home/sbu/odoo/odoo/addons/base/models/ir_http.py", line 229, in _dispatch
    result = request.dispatch()
  File "/home/sbu/odoo/odoo/http.py", line 813, in dispatch
    r = self._call_function(**self.params)
  File "/home/sbu/odoo/odoo/http.py", line 353, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/sbu/odoo/odoo/service/model.py", line 93, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/sbu/odoo/odoo/http.py", line 349, in checked_call
    result.flatten()
  File "/home/sbu/odoo/odoo/http.py", line 1240, in flatten
    self.response.append(self.render())
  File "/home/sbu/odoo/odoo/http.py", line 1233, in render
    return env["ir.ui.view"].render_template(self.template, self.qcontext)
  File "/home/sbu/odoo/odoo/addons/base/models/ir_ui_view.py", line 1602, in render_template
    return self.browse(self.get_view_id(template)).render(values, engine)
  File "/home/sbu/odoo/addons/website/models/ir_ui_view.py", line 417, in render
    return super(View, self).render(values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "/home/sbu/odoo/addons/web_editor/models/ir_ui_view.py", line 26, in render
    return super(IrUiView, self).render(values=values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "/home/sbu/odoo/odoo/addons/base/models/ir_ui_view.py", line 1610, in render
    return self.env[engine].render(self.id, qcontext)
  File "/home/sbu/odoo/odoo/addons/base/models/ir_qweb.py", line 58, in render
    result = super(IrQWeb, self).render(id_or_xml_id, values=values, **context)
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 256, in render
    self.compile(template, options)(self, body.append, values or {})
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 330, in _compiled_fn
    raise e
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 328, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_survey_survey_page_statistics_1171
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 335, in _compiled_fn
    raise QWebException("Error to render compiling AST", e, path, node and etree.tostring(node[0], encoding='unicode'), name)
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/sbu/odoo/odoo/addons/base/models/ir_http.py", line 229, in _dispatch
    result = request.dispatch()
  File "/home/sbu/odoo/odoo/http.py", line 813, in dispatch
    r = self._call_function(**self.params)
  File "/home/sbu/odoo/odoo/http.py", line 353, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/sbu/odoo/odoo/service/model.py", line 93, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/sbu/odoo/odoo/http.py", line 349, in checked_call
    result.flatten()
  File "/home/sbu/odoo/odoo/http.py", line 1240, in flatten
    self.response.append(self.render())
  File "/home/sbu/odoo/odoo/http.py", line 1233, in render
    return env["ir.ui.view"].render_template(self.template, self.qcontext)
  File "/home/sbu/odoo/odoo/addons/base/models/ir_ui_view.py", line 1602, in render_template
    return self.browse(self.get_view_id(template)).render(values, engine)
  File "/home/sbu/odoo/addons/website/models/ir_ui_view.py", line 417, in render
    return super(View, self).render(values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "/home/sbu/odoo/addons/web_editor/models/ir_ui_view.py", line 26, in render
    return super(IrUiView, self).render(values=values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "/home/sbu/odoo/odoo/addons/base/models/ir_ui_view.py", line 1610, in render
    return self.env[engine].render(self.id, qcontext)
  File "/home/sbu/odoo/odoo/addons/base/models/ir_qweb.py", line 58, in render
    result = super(IrQWeb, self).render(id_or_xml_id, values=values, **context)
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 256, in render
    self.compile(template, options)(self, body.append, values or {})
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 330, in _compiled_fn
    raise e
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 328, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_survey_survey_page_statistics_1171
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 335, in _compiled_fn
    raise QWebException("Error to render compiling AST", e, path, node and etree.tostring(node[0], encoding='unicode'), name)
odoo.addons.base.models.qweb.QWebException: 'NoneType' object has no attribute 'is_session_answer'
Traceback (most recent call last):
  File "/home/sbu/odoo/odoo/addons/base/models/qweb.py", line 328, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_survey_layout_1303
AttributeError: 'NoneType' object has no attribute 'is_session_answer'

Error to render compiling AST
AttributeError: 'NoneType' object has no attribute 'is_session_answer'
Template: survey.layout
Path: /t/html/body/div/div[2]/div[2]/t[3]
Node: <t t-set="page_per_question_back_button" t-value="can_go_back and survey.questions_layout == 'page_per_question' and not answer.is_session_answer and question != answer.predefined_question_ids[0] and (not survey.page_ids or question != survey.page_ids[0])"/>