Can someone help me trace back what is breaking my ability to view Models in Settings > Database Structure > Models

I receive this Odoo Server Error whenever I try to open up and Models

Traceback (most recent call last): File "/opt/odoo/odoo-server/openerp/http.py", line 525, in _handle_exception return super(JsonRequest, self)._handle_exception(exception) File "/opt/odoo/odoo-server/openerp/http.py", line 562, in dispatch result = self._call_function(**self.params) File "/opt/odoo/odoo-server/openerp/http.py", line 302, in _call_function return checked_call(self.db, *args, **kwargs) File "/opt/odoo/odoo-server/openerp/service/model.py", line 113, in wrapper return f(dbname, *args, **kwargs) File "/opt/odoo/odoo-server/openerp/http.py", line 299, in checked_call return self.endpoint(*a, **kw) File "/opt/odoo/odoo-server/openerp/http.py", line 791, in __call__ return self.method(*args, **kw) File "/opt/odoo/odoo-server/openerp/http.py", line 395, in response_wrap response = f(*args, **kw) File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 949, in call_kw return self._call_kw(model, method, args, kwargs) File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 932, in _call_kw records = getattr(request.session.model(model), method)(*args, **kwargs) File "/opt/odoo/odoo-server/openerp/http.py", line 895, in proxy result = meth(cr, request.uid, *args, **kw) File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo/odoo-server/addons/calendar/calendar.py", line 631, in read data = super(ir_model, self).read(cr, uid, new_ids, fields=fields, context=context, load=load) File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper return old_api(self, *args, **kwargs) File "/opt/odoo/odoo-server/openerp/models.py", line 3094, in read result = BaseModel.read(records, fields, load=load) File "/opt/odoo/odoo-server/openerp/api.py", line 239, in wrapper return new_api(self, *args, **kwargs) File "/opt/odoo/odoo-server/openerp/models.py", line 3126, in read self._read_from_database(stored) File "/opt/odoo/odoo-server/openerp/api.py", line 239, in wrapper return new_api(self, *args, **kwargs) File "/opt/odoo/odoo-server/openerp/models.py", line 3290, in _read_from_database res2 = self._columns[f].get(cr, self._model, ids, f, user, context=context, values=result) File "/opt/odoo/odoo-server/openerp/osv/fields.py", line 725, in get records = comodel.search(domain, limit=self._limit) File "/opt/odoo/odoo-server/openerp/api.py", line 239, in wrapper return new_api(self, *args, **kwargs) TypeError: 'NoneType' object is not callable