[NR-308026] Fix Null pointer exception while shutting down harvester

[ShivanshPrakash]

While calling Harvest's shutdown method, the current checks only consider the nullability of the Harvest instance and it's harvester. But, the harvestTimer is also accessed during the method execution. If the harvestTimer is null due to some reason, for instance, a back to back call to the shutdown method, we get a null pointer exception.

Added a null check to prevent this.

This Null pointer exception is one of the top crashes happening in our application. Also attached the stacktrace below.

Exception java.lang.NullPointerException: Attempt to invoke virtual method 'void com.newrelic.agent.android.harvest.HarvestTimer.shutdown()' on a null object reference
  at com.newrelic.agent.android.harvest.Harvest.shutdownHarvester (Harvest.java:119)
  at com.newrelic.agent.android.harvest.Harvest.shutdown (Harvest.java:129)
  at com.newrelic.agent.android.AndroidAgentImpl.stop (AndroidAgentImpl.java:548)
  at com.newrelic.agent.android.AndroidAgentImpl.stop (AndroidAgentImpl.java:473)
  at com.newrelic.agent.android.AndroidAgentImpl.applicationBackgrounded (AndroidAgentImpl.java:629)
  at com.newrelic.agent.android.background.ApplicationStateMonitor.notifyApplicationInBackground (ApplicationStateMonitor.java:108)
  at com.newrelic.agent.android.background.ApplicationStateMonitor$1.run (ApplicationStateMonitor.java:65)
  at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1145)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:644)
  at java.lang.Thread.run (Thread.java:1012)

[CLAassistant]

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[ShivanshPrakash]

@cthomas-newrelic Can you please have a look or assign someone to review this?

[ywang-nr]

@ShivanshPrakash Thanks for the PR, we will include this PR in the next sprint. Ticket here, will be included in v7.6.0.