Fix NPE being thrown intermittently while removing bundles in shutdown process

[pcnfernando]

Purpose

Fixes #1540

[codecov-io]

Codecov Report

Merging #1541 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #1541      +/-   ##
============================================
- Coverage     67.93%   67.89%   -0.04%     
  Complexity       45       45              
============================================
  Files           648      648              
  Lines         27313    27315       +2     
  Branches       4296     4297       +1     
============================================
- Hits          18554    18545       -9     
- Misses         6965     6971       +6     
- Partials       1794     1799       +5

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/siddhi/core/util/SiddhiExtensionLoader.java	46.66% <0%> (-0.71%)	0 <0> (ø)
.../siddhi/core/util/event/handler/StreamHandler.java	62% <0%> (-12%)	0% <0%> (ø)
...java/io/siddhi/core/stream/input/InputHandler.java	69.44% <0%> (-5.56%)	0% <0%> (ø)
...ut/stream/state/AbsentStreamPreStateProcessor.java	85.51% <0%> (-1.38%)	0% <0%> (ø)
...o/siddhi/core/aggregation/IncrementalExecutor.java	80.46% <0%> (-0.79%)	0% <0%> (ø)
.../io/siddhi/core/util/snapshot/SnapshotService.java	71.22% <0%> (+0.41%)	0% <0%> (ø)	⬇️

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