Add fix for possible NPE

[mohanvive]

Purpose

$subject...

[codecov-io]

Codecov Report

Merging #1577 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master   #1577      +/-   ##
===========================================
- Coverage      67.7%   67.7%   -0.01%     
  Complexity       45      45              
===========================================
  Files           650     650              
  Lines         27471   27471              
  Branches       4330    4330              
===========================================
- Hits          18600   18599       -1     
  Misses         7059    7059              
- Partials       1812    1813       +1

Impacted Files	Coverage Δ	Complexity Δ
.../siddhi/core/stream/input/source/SourceMapper.java	54.92% <0%> (ø)	0 <0> (ø)	⬇️
...ut/stream/state/AbsentStreamPreStateProcessor.java	82.11% <0%> (-1.33%)	0% <0%> (ø)
.../io/siddhi/core/util/snapshot/SnapshotService.java	70.6% <0%> (-0.83%)	0% <0%> (ø)
...o/siddhi/core/aggregation/IncrementalExecutor.java	80.46% <0%> (-0.79%)	0% <0%> (ø)
.../siddhi/core/util/event/handler/StreamHandler.java	74% <0%> (+12%)	0% <0%> (ø)	⬇️

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