EVENT_TIME timer throws exception when side input used

Receive this exception when using a DoFn containing an EVENT_TIME timer and passing a side input. Pipeline processing aborts. It works as expected when the side input is absent.
```

2018-11-06-12:03:32 ERROR o.a.b.r.direct.QuiescenceDriver - Internal Error while delivering timers
java.lang.IllegalArgumentException:
expected one element but was:
at org.apache.beam.repackaged.beam_runners_direct_java.com.google.common.collect.Iterators.getOnlyElement(Iterators.java:322)

at org.apache.beam.repackaged.beam_runners_direct_java.com.google.common.collect.Iterables.getOnlyElement(Iterables.java:294)

at org.apache.beam.runners.direct.QuiescenceDriver.fireTimers(QuiescenceDriver.java:167)
at org.apache.beam.runners.direct.QuiescenceDriver.drive(QuiescenceDriver.java:110)

at org.apache.beam.runners.direct.ExecutorServiceParallelExecutor$2.run(ExecutorServiceParallelExecutor.java:170)

at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)

at java.lang.Thread.run(Thread.java:748)

```

Imported from Jira [BEAM-5997](https://issues.apache.org/jira/browse/BEAM-5997). Original Jira may contain additional context.
Reported by: laurallew.