This error got introduced on CRM 2015 after upgrading to 1.0.
unable to get property ‘$o_3’ of undefined or null reference
This error caused due to unsupported customization in the Java script on form load. The new form rendering mechanism does not recognise this script. using document.getElementById is unsupported. If you are using pre-filter default view on form load, this error will pop-up.
Note that this error only occurs when you turn on the “Use Legacy form rendering”
You only need to turn on this form rendering, when there is lot of JS functionality is used and not behaving in intended manner.
If you are trying to add a custom view to the lookup and set that as the default, then you can use the
Xrm.Page.ui.controls.get(“[tablename]”).addCustomView(viewID, “[tablename]”, “[viewname]”, “[fetchXMLString]”, “[TSAlayoutXMLstring]”, true);