Grails Testing Plugin often occurred exception

In previous posts (service unit testing and domain entity unit testing) I told You a lot about Grails Testing Plugin, its simple usage, but have not paid necessary accent on one often occurred exception:

"Cannot invoke method containsKey() on null object" type="java.lang.NullPointerException"

It happens then You override setUp() method but forget to call super method at first. So instead of writing:

class SomeTests extends GrailsUnitTestCase {

   void setUp() {
       // Some initializations.
   }

}

Call super method at first:

class SomeTests extends GrailsUnitTestCase {

    void setUp() {
        
        // Call super method at first.
        super.setUp()
 
        
        // Some initializations.
    }

}

That's all.