[CMT2][BUG] After some operation, it will pop NullPointerException

1 Use the xe in attachment to do XE migration
2 Goto step 3, choose the tables as attachment, choosetables.png
4 Goto step4

Actual result
It will pop error as attachment.

another situation:
1.Use "New XE migration", select the file:xmltonew.xml(refer to the attachment).
2.go to step1, change the xml file to "test.xml"(refer to the attachment).
3.go to step3, select another two tables:agent and agent1,then click "Next"(refer to the attachment)
4.go to step4, you will see the error window.(refer to the attachment)