Fixed delgrp bug

[neoruien]

No description provided.

[codecov-io]

Codecov Report

Merging #185 into master will decrease coverage by 0.33%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #185      +/-   ##
============================================
- Coverage     47.72%   47.38%   -0.34%     
  Complexity      765      765              
============================================
  Files           155      155              
  Lines          3795     3822      +27     
  Branches        473      477       +4     
============================================
  Hits           1811     1811              
- Misses         1840     1867      +27     
  Partials        144      144              

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/team/serenity/model/ModelManager.java	52.10% <0.00%> (-0.56%)	31.00 <0.00> (ø)
...java/team/serenity/model/group/GroupLessonKey.java	69.23% <0.00%> (-5.77%)	3.00 <0.00> (ø)
...va/team/serenity/model/managers/LessonManager.java	78.57% <0.00%> (-4.45%)	17.00 <0.00> (ø)
.../team/serenity/model/managers/QuestionManager.java	82.22% <0.00%> (-17.78%)	18.00 <0.00> (ø)
...am/serenity/model/managers/StudentInfoManager.java	75.00% <0.00%> (-15.70%)	15.00 <0.00> (ø)
...a/team/serenity/model/managers/StudentManager.java	76.47% <0.00%> (-4.78%)	15.00 <0.00> (ø)
src/main/java/team/serenity/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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