SObject row was retrieved via SOQL without querying the requested field in controller extension

A standardcontroller's record will only contain the fields referenced in the visualforce page you're using it on. The standard controller addFields(List) method (docs) allows you to extend this to the fields you need in your apex code.

cont.addFields(new List<String>{'Assigned_to_Partner__c'});

Set this as the 2nd line in your constructor.

A field has to be on the page to be included - so as well as using addfields, you can also just add the field in an output panel with rendered set to false - it should then be included in the SOQL