When working with Xtext you might hit this issue at some point. The rule ‘Primary’ may be consumed without object instantiation. Add an action to ensure object creation, e.g. ‘{Expression}’.

You will have something similar in your Xtext file:
Primary returns Expression:
'modify'
Solution
Use explicit notation instantiate with {}
Primary returns Expression:
{Modify}'modify'
For more information go to Xtext documentation https://www.eclipse.org/Xtext/documentation/301_grammarlanguage.html#syntax Section: Simple Actions. It will create an instance of Modify and assign it to the result of the parser rule.