Major Description
Current Stage page's task table will throw an exception when there's no complete tasks, to reproduce this issue, user could submit code like:
sc.parallelize(1 to 20, 20).map { i => Thread.sleep(10000); i }.collect()
Then open the UI and click into stage details. Below is the screenshot.
Deep dive into the code, found that current UI can only show the completed tasks, it is different from 2.2 code.
