A query like 

```
sequence by user.name with maxspan=90m
![ authentication where ... ]
[ authentication where ...]
![ authentication where ... ]
[ authentication where ... ]
![ authentication where ... ]
```

can result in a NullPointerException

```
java.lang.NullPointerException: Cannot invoke "org.elasticsearch.xpack.eql.execution.sequence.Match.ordinal()" because the return value of "org.elasticsearch.xpack.eql.execution.sequence.Sequence.matchAt(int)" is null
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.prepareQueryForMissingEvents(TumblingWindow.java:313)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.checkMissingEvents(TumblingWindow.java:219)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.payload(TumblingWindow.java:717)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.completeBaseCriterion(TumblingWindow.java:499)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.baseCriterion(TumblingWindow.java:405)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.lambda$advance$6(TumblingWindow.java:356)
```
x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/execution/sequence/SequenceMatcher.java
private void clearCircuitBreaker() {
        circuitBreaker.addWithoutBreaking(-prevRamBytesUsedInFlight - prevRamBytesUsedCompleted);
        prevRamBytesUsedInFlight = 0;
        prevRamBytesUsedCompleted = 0;
    }
    
    
    
    
    
    private void trackMemory() {
        long newRamBytesUsedInFlight = ramBytesUsedInFlight();
        circuitBreaker.addEstimateBytesAndMaybeBreak(newRamBytesUsedInFlight - prevRamBytesUsedInFlight, CB_INFLIGHT_LABEL);
        prevRamBytesUsedInFlight = newRamBytesUsedInFlight;
        long newRamBytesUsedCompleted = ramBytesUsedCompleted();
        circuitBreaker.addEstimateBytesAndMaybeBreak(newRamBytesUsedCompleted - prevRamBytesUsedCompleted, CB_COMPLETED_LABEL);
        prevRamBytesUsedCompleted = newRamBytesUsedCompleted;
    }
    @Override
    public String toString() {
        return LoggerMessageFormat.format(
            null,
            "Tracking [{}] keys with [{}] completed and {} in-flight",
            keyToSequences,
            completed.size(),
            stageToKeys
        );
    }
}