[incubator-uniffle] 03/17: [Bugfix] Fix spark2 executor stop NPE problem (#186)

[jshao]

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commit 924dac7f093d0b3f581e521fc71bc30ea0963907
Author: roryqi <jerqi1242949...@gmail.com>
AuthorDate: Wed Jun 22 14:34:06 2022 +0800

    [Bugfix] Fix spark2 executor stop NPE problem (#186)
    
    ### What changes were proposed in this pull request?
    We need to judge heartbeatExecutorService whether is null when we will stop 
it.
    
    ### Why are the changes needed?
    #177 pr introduce this problem, when we run Spark applications on our 
cluster, the executor will throw NPE when method `stop` is called.
    
    ### Does this PR introduce _any_ user-facing change?
    No
    
    ### How was this patch tested?
    Manual test