Custom Code Validation Tool version and best practices for browser errors

Hi 

We are using Dynamics CRM 2013 on Premise version. 

We are having browser(IE only) issues like

1. "Can't move focus to the control because it is invisible, not enabled, or of a type that does not accept the focus"

2. "null is null not an object." 

So in one of the forums says we can run custom code validation tool to know the issues. 

So do we have to run 

Microsoft Dynamics CRM 2015 Custom Code Validation Tool  or 

Microsoft Dynamics CRM 2013 Custom Code Validation Tool

and also best practices to avoid these kind of errors.