You may get error "object reference not to set to an instance of an object" while saving the build definition after upgrade from Build server from 2010 to 2012
One of the scenario where you may face this problem after upgrade is; if you have set the version control assembly path in build controller properties in 2010.
Hence, in order to resolve this issue, remove that path and set it again and restart the build controller service.
Note: You will see this problem on the client machines where you have VS2010 and VS2012 clients. If you don’t have 2010, then you will not notice it.
Written by: Deepak Mittal, Support Escalation Engineer
Reviewed by: George Archer(Senior Escalation Engineer) and Nitish Nagpal(Support Escalation Engineer)