SPARK-4175. Exception on stage page

[sryza]

No description provided.

[JoshRosen]

/cc @kayousterhout are there other places where we need to update this?

[SparkQA]

Test build #22650 has started for PR 3043 at commit e327340.

• This patch merges cleanly.

[kayousterhout]

LGTM -- I'll merge this when the tests finish. Thanks for fixing this Sandy!

[kayousterhout]

@JoshRosen I think this is the only place where it's broken; all other uses of metrics.get that I added are in creating the summary table, and there, the tasks without task metrics have already been filtered out.

[andrewor14]

LGTM. At some point we should try to understand why metrics can be None sometimes. I've noticed many entries in a task row going blank before.

[SparkQA]

Test build #22650 has finished for PR 3043 at commit e327340.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/22650/
Test PASSed.

[kayousterhout]

github seems to be slow to update, but I've merged this into master. Thanks Sandy!!