[KYUUBI-118]fix npe bug with incremental result collection

[yaooqinn]

No description provided.

[codecov]

Codecov Report

Merging #119 into master will increase coverage by 0.31%.
The diff coverage is 92.1%.

@@             Coverage Diff              @@
##             master     #119      +/-   ##
============================================
+ Coverage     72.24%   72.56%   +0.31%     
- Complexity      529      542      +13     
============================================
  Files            72       72              
  Lines          3423     3266     -157     
  Branches        491      475      -16     
============================================
- Hits           2473     2370     -103     
+ Misses          676      639      -37     
+ Partials        274      257      -17

Impacted Files	Coverage Δ	Complexity Δ
...ver/src/main/scala/org/apache/spark/SparkEnv.scala	76.92% <ø> (+8.17%)	0 <0> (ø)	⬇️
...la/yaooqinn/kyuubi/spark/SparkSessionWithUGI.scala	75% <100%> (+0.24%)	17 <9> (+5)	⬆️
...scala/yaooqinn/kyuubi/session/SessionManager.scala	77.27% <100%> (ø)	28 <0> (ø)	⬇️
...a/yaooqinn/kyuubi/operation/OperationManager.scala	79.03% <100%> (ø)	28 <0> (ø)	⬇️
...la/yaooqinn/kyuubi/operation/KyuubiOperation.scala	55.91% <50%> (ø)	39 <0> (ø)	⬇️
.../scala/yaooqinn/kyuubi/session/KyuubiSession.scala	72.13% <90.9%> (+0.23%)	44 <10> (+8)	⬆️
...ain/scala/yaooqinn/kyuubi/KyuubiSQLException.scala	95.45% <93.75%> (+0.85%)	8 <1> (ø)	⬇️

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