AttributeError: 'NoneType' object has no attribute 'fields_get' in Openerp

[mal920]

why does this error occurs? is anyone familiar with this error. i got this error when i click any of the tabs eg: sales,purchase,human resource,accounting etc.\[code\]Traceback (most recent call last):File "/home/openERP/src/openerp-server/bin/osv/osv.py", line 122, in wrapperreturn f(self, dbname, *args, **kwargs)File "/home/openERP/src/openerp-server/bin/osv/osv.py", line 176, in executeres = self.execute_cr(cr, uid, obj, method, *args, **kw)File "/home/openERP/src/openerp-server/bin/osv/osv.py", line 167, in execute_crreturn getattr(object, method)(cr, uid, *args, **kw)File "/home/openERP/src/openerp-server/bin/addons/hr/hr_department.py", line 94, in readres = super(ir_action_window, self).read(cr, uid, select, fields=fields, context=context, load=load)File "/home/openERP/src/openerp-server/bin/osv/orm.py", line 2944, in readresult = self._read_flat(cr, user, select, fields, context, load)File "/home/openERP/src/openerp-server/bin/osv/orm.py", line 3064, in _read_flatres2 = self._columns[f].get(cr, self, ids, f, user, context=context, values=res)File "/home/openERP/src/openerp-server/bin/osv/fields.py", line 800, in getres = self._fnct(obj, cr, user, ids, name, self._arg, context)File "/home/openERP/src/openerp-server/bin/addons/base/ir/ir_actions.py", line 193, in _search_viewfields_from_fields_get = self.pool.get(act.res_model).fields_get(cr, uid, context=context)AttributeError: 'NoneType' object has no attribute 'fields_get'\[/code\]