DataSource definition error "Cannot invoke "

Hi 

I am using ReadyApi to test RestAPI and in test cases i used to define DataSource to define variables and mostly choose DataGenerator, So in first time definition it works  and i add some properties &save them but when i came back to it to add another property and try to Run it to see values in data log list , usually i face with below error :

Cannot invoke "com.eviware.soapui.impl.wsdl.teststeps.datasource.DataGenerator.GeneratedDataValue.getValue()" because the return value of "java.util.Map.get(Object)" is null

So does any one have similar experience ? 

Do you have any idea on how to solve it !?