Getting org.eclipse.core.runtime.AssertionFailedException while running the plain JAVA API Application

I am developing an application to generate Report in the CSV file,using plain java API.I have to filter the work item of type task.I have stored the list of the work Item into the Array list,and pass this Array list to another class  to find each work item related data.My code is giving exact output,but in one of the case it is not giving output as per expectation. Whenever the resultant list of work item is  more,suppose 20,then it is throwing Exception as above"org.eclipse.core.runtime.AssertionFailedException".I debug the code ,from there I comes to know that Array list is having 3 parameter- 

variable                                       value      

element data                               object[33]

mod count                                     28

size                                                28

Resultant work item is 104,but the Work item which satisfies the condition is 28.when I am printing the data into console it is showing perfectly 28 list of work item,but in debug perspective it is showing 33 list of Work item.After 28 ,other list of work item are null.IF  total work item is 28,then I am getting the output of only 1 work item.

I am not getting,where I did mistake .I liked to know why null is passing to the Array list. Please help me to resolve this issue.