Symptoms

When trying to use "execute another rule" Action on a form you may receive an error:

Widget Error:t.selectSingleNode(...) is null

Error occurs in IE, Firefox and Chrome. After this error rule list is not being loaded so that it is not possible to select any rule. This happens on one specific form.
 

Diagnoses

It can be the case that the problematic rule on the form executes number of SMO Method calls (one after the other) when a button on the form is clicked and these SMOs are bound and retrieve data from a Web Service. In such case this error can be caused by the fact that certain changes have been applied in the WebService(s), but the SMO(s) have not been aligned with these. In order to fix this you have to change/refresh the SMO(s) accordingly.

In some other scenarios where you receiving this error you may try and re-add views on the affected form (if number of views allows you to easily try this).
 

Resolution

See possible solutions above.