java.lang.NullPointerException when sorting project properties

I have 146 project properties that I would like to sort. I have found this post. I modified the code a bit to sort project properties. See below: 

def obj = testRunner.testCase.testSuite.project
log.info "Number of Properties: " + obj.getPropertyCount();

def propList = []; // Arraylist to store properties
obj.getPropertyList().each { prop -> // For all Project (custom) properties
	propList.add(prop.name);  // Add it to the arraylist
}
def sortedList = propList.sort(); // Sort the array list
def index = 0
sortedList.each { name -> 	// Loop the sorted ArrayList
	log.info index + " - " + name + " - " + obj.getPropertyValue(name)
	obj.moveProperty(name, index); // Get Project (custom) property by name an move it to sorted position
	index++
}

I randomly get "java.lang.NullPointerException" when running the code at red statement above. I does however sort 4 to 10 properties before I get the error. So each time I re-run the code I get a bit further before I get error message. So I managed to get the properties sorted. 

But why am I getting this error?

Thanks in advance!