Hello, when I click html/css editor I get the following error. how can i solve it?


Server application error
Error code: 200
Error message: Odoo Server Error
Error data message:
Expected singleton: \ir\.ui\.view\(171\,\\ 1540\)\ \
\ Error\ data\ debug\:\
\ Traceback\ \(most\ recent\ call\ last\)\:\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/models\.py\"\,\ line\ 4718\,\ in\ ensure_one\
\ \ \ \ _id\,\ \=\ self\._ids\
ValueError\:\ too\ many\ values\ to\ unpack\ \(expected\ 1\)\
\
During\ handling\ of\ the\ above\ exception\,\ another\ exception\ occurred\:\
\
Traceback\ \(most\ recent\ call\ last\)\:\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 656\,\ in\ _handle_exception\
\ \ \ \ return\ super\(JsonRequest\,\ self\)\._handle_exception\(exception\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 314\,\ in\ _handle_exception\
\ \ \ \ raise\ pycompat\.reraise\(type\(exception\)\,\ exception\,\ sys\.exc_info\(\)\[2\]\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/tools\/pycompat\.py\"\,\ line\ 87\,\ in\ reraise\
\ \ \ \ raise\ value\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 698\,\ in\ dispatch\
\ \ \ \ result\ \=\ self\._call_function\(\*\*self\.params\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 346\,\ in\ _call_function\
\ \ \ \ return\ checked_call\(self\.db\,\ \*args\,\ \*\*kwargs\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/service\/model\.py\"\,\ line\ 97\,\ in\ wrapper\
\ \ \ \ return\ f\(dbname\,\ \*args\,\ \*\*kwargs\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 339\,\ in\ checked_call\
\ \ \ \ result\ \=\ self\.endpoint\(\*a\,\ \*\*kw\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 941\,\ in\ __call__\
\ \ \ \ return\ self\.method\(\*args\,\ \*\*kw\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/http\.py\"\,\ line\ 519\,\ in\ response_wrap\
\ \ \ \ response\ \=\ f\(\*args\,\ \*\*kw\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/addons\/web_editor\/controllers\/main\.py\"\,\ line\ 287\,\ in\ get_assets_editor_resources\
\ \ \ \ views\ \=\ request\.env\[\"\\ir\.ui\.view\"\]\.get_related_views\(key\,\\ bundles\=bundles\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/addons\/website\/models\/ir_ui_view\.py\"\,\ line\ 185\,\ in\ get_related_views\
\ \ \ \ return\ super\(View\,\ self\)\.get_related_views\(key\,\ bundles\=bundles\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/addons\/web_editor\/models\/ir_ui_view\.py\"\,\ line\ 276\,\ in\ get_related_views\
\ \ \ \ views\ \=\ View\._views_get\(key\,\ bundles\=bundles\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/addons\/web_editor\/models\/ir_ui_view\.py\"\,\ line\ 253\,\ in\ _views_get\
\ \ \ \ views_to_return\ \+\=\ self\._views_get\(called_view\,\ options\=options\,\ bundles\=bundles\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/addons\/web_editor\/models\/ir_ui_view\.py\"\,\ line\ 253\,\ in\ _views_get\
\ \ \ \ views_to_return\ \+\=\ self\._views_get\(called_view\,\ options\=options\,\ bundles\=bundles\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/addons\/web_editor\/models\/ir_ui_view\.py\"\,\ line\ 238\,\ in\ _views_get\
\ \ \ \ while\ root\ and\ view\.inherit_id\:\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/fields\.py\"\,\ line\ 979\,\ in\ __get__\
\ \ \ \ record\.ensure_one\(\)\
\ \ File\ \"\/home\/odoo\/odoo12\/odoo\/odoo\/models\.py\"\,\ line\ 4721\,\ in\ ensure_one\
\ \ \ \ raise\ ValueError\(\"Expected\ singleton\:\ \%s\"\ \%\ self\)\
ValueError\:\ Expected\ singleton\:\ \ir.ui.view(171, 1540)